[SPARK-14498][ML][PYTHON][SQL] Many cleanups to ML and ML-related docs

## What changes were proposed in this pull request?

Cleanups to documentation.  No changes to code.
* GBT docs: Move Scala doc for private object GradientBoostedTrees to public docs for GBTClassifier,Regressor
* GLM regParam: needs doc saying it is for L2 only
* TrainValidationSplitModel: add .. versionadded:: 2.0.0
* Rename “_transformer_params_from_java” to “_transfer_params_from_java”
* LogReg Summary classes: “probability” col should not say “calibrated”
* LR summaries: coefficientStandardErrors —> document that intercept stderr comes last.  Same for t,p-values
* approxCountDistinct: Document meaning of “rsd" argument.
* LDA: note which params are for online LDA only

## How was this patch tested?

Doc build

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #12266 from jkbradley/ml-doc-cleanups.

12 files changed, 97 insertions, 42 deletions

diff --git a/mllib/src/main/scala/org/apache/spark/ml/classification/GBTClassifier.scala b/mllib/src/main/scala/org/apache/spark/ml/classification/GBTClassifier.scala
index bee90fb3a5..a2150fbcc3 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/classification/GBTClassifier.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/classification/GBTClassifier.scala
@@ -42,6 +42,16 @@ import org.apache.spark.sql.functions._
  * learning algorithm for classification.
  * It supports binary labels, as well as both continuous and categorical features.
  * Note: Multiclass labels are not currently supported.
+ *
+ * The implementation is based upon: J.H. Friedman. "Stochastic Gradient Boosting." 1999.
+ *
+ * Notes on Gradient Boosting vs. TreeBoost:
+ *  - This implementation is for Stochastic Gradient Boosting, not for TreeBoost.
+ *  - Both algorithms learn tree ensembles by minimizing loss functions.
+ *  - TreeBoost (Friedman, 1999) additionally modifies the outputs at tree leaf nodes
+ *    based on the loss function, whereas the original gradient boosting method does not.
+ *  - We expect to implement TreeBoost in the future:
+ *    [https://issues.apache.org/jira/browse/SPARK-4240]
  */
 @Since("1.4.0")
 @Experimental
diff --git a/mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala b/mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala
index 37182928cc..268c3e32c3 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala
@@ -774,10 +774,10 @@ sealed trait LogisticRegressionTrainingSummary extends LogisticRegressionSummary
  */
 sealed trait LogisticRegressionSummary extends Serializable {
 
-  /** Dataframe outputted by the model's `transform` method. */
+  /** Dataframe output by the model's `transform` method. */
   def predictions: DataFrame
 
-  /** Field in "predictions" which gives the calibrated probability of each class as a vector. */
+  /** Field in "predictions" which gives the probability of each class as a vector. */
   def probabilityCol: String
 
   /** Field in "predictions" which gives the true label of each instance (if available). */
@@ -792,8 +792,8 @@ sealed trait LogisticRegressionSummary extends Serializable {
  * :: Experimental ::
  * Logistic regression training results.
  *
- * @param predictions dataframe outputted by the model's `transform` method.
- * @param probabilityCol field in "predictions" which gives the calibrated probability of
+ * @param predictions dataframe output by the model's `transform` method.
+ * @param probabilityCol field in "predictions" which gives the probability of
  *                       each class as a vector.
  * @param labelCol field in "predictions" which gives the true label of each instance.
  * @param featuresCol field in "predictions" which gives the features of each instance as a vector.
@@ -816,8 +816,8 @@ class BinaryLogisticRegressionTrainingSummary private[classification] (
  * :: Experimental ::
  * Binary Logistic regression results for a given model.
  *
- * @param predictions dataframe outputted by the model's `transform` method.
- * @param probabilityCol field in "predictions" which gives the calibrated probability of
+ * @param predictions dataframe output by the model's `transform` method.
+ * @param probabilityCol field in "predictions" which gives the probability of
  *                       each class as a vector.
  * @param labelCol field in "predictions" which gives the true label of each instance.
  * @param featuresCol field in "predictions" which gives the features of each instance as a vector.
diff --git a/mllib/src/main/scala/org/apache/spark/ml/clustering/LDA.scala b/mllib/src/main/scala/org/apache/spark/ml/clustering/LDA.scala
index 727b724708..89a7a4ccf6 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/clustering/LDA.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/clustering/LDA.scala
@@ -190,6 +190,8 @@ private[clustering] trait LDAParams extends Params with HasFeaturesCol with HasM
   def getTopicDistributionCol: String = $(topicDistributionCol)
 
   /**
+   * For Online optimizer only: [[optimizer]] = "online".
+   *
    * A (positive) learning parameter that downweights early iterations. Larger values make early
    * iterations count less.
    * This is called "tau0" in the Online LDA paper (Hoffman et al., 2010)
@@ -198,8 +200,9 @@ private[clustering] trait LDAParams extends Params with HasFeaturesCol with HasM
    * @group expertParam
    */
   @Since("1.6.0")
-  final val learningOffset = new DoubleParam(this, "learningOffset", "A (positive) learning" +
-    " parameter that downweights early iterations. Larger values make early iterations count less.",
+  final val learningOffset = new DoubleParam(this, "learningOffset", "(For online optimizer)" +
+    " A (positive) learning parameter that downweights early iterations. Larger values make early" +
+    " iterations count less.",
     ParamValidators.gt(0))
 
   /** @group expertGetParam */
@@ -207,6 +210,8 @@ private[clustering] trait LDAParams extends Params with HasFeaturesCol with HasM
   def getLearningOffset: Double = $(learningOffset)
 
   /**
+   * For Online optimizer only: [[optimizer]] = "online".
+   *
    * Learning rate, set as an exponential decay rate.
    * This should be between (0.5, 1.0] to guarantee asymptotic convergence.
    * This is called "kappa" in the Online LDA paper (Hoffman et al., 2010).
@@ -215,15 +220,17 @@ private[clustering] trait LDAParams extends Params with HasFeaturesCol with HasM
    * @group expertParam
    */
   @Since("1.6.0")
-  final val learningDecay = new DoubleParam(this, "learningDecay", "Learning rate, set as an" +
-    " exponential decay rate. This should be between (0.5, 1.0] to guarantee asymptotic" +
-    " convergence.", ParamValidators.gt(0))
+  final val learningDecay = new DoubleParam(this, "learningDecay", "(For online optimizer)" +
+    " Learning rate, set as an exponential decay rate. This should be between (0.5, 1.0] to" +
+    " guarantee asymptotic convergence.", ParamValidators.gt(0))
 
   /** @group expertGetParam */
   @Since("1.6.0")
   def getLearningDecay: Double = $(learningDecay)
 
   /**
+   * For Online optimizer only: [[optimizer]] = "online".
+   *
    * Fraction of the corpus to be sampled and used in each iteration of mini-batch gradient descent,
    * in range (0, 1].
    *
@@ -239,8 +246,9 @@ private[clustering] trait LDAParams extends Params with HasFeaturesCol with HasM
    * @group param
    */
   @Since("1.6.0")
-  final val subsamplingRate = new DoubleParam(this, "subsamplingRate", "Fraction of the corpus" +
-    " to be sampled and used in each iteration of mini-batch gradient descent, in range (0, 1].",
+  final val subsamplingRate = new DoubleParam(this, "subsamplingRate", "(For online optimizer)" +
+    " Fraction of the corpus to be sampled and used in each iteration of mini-batch" +
+    " gradient descent, in range (0, 1].",
     ParamValidators.inRange(0.0, 1.0, lowerInclusive = false, upperInclusive = true))
 
   /** @group getParam */
@@ -248,6 +256,8 @@ private[clustering] trait LDAParams extends Params with HasFeaturesCol with HasM
   def getSubsamplingRate: Double = $(subsamplingRate)
 
   /**
+   * For Online optimizer only (currently): [[optimizer]] = "online".
+   *
    * Indicates whether the docConcentration (Dirichlet parameter for
    * document-topic distribution) will be optimized during training.
    * Setting this to true will make the model more expressive and fit the training data better.
@@ -257,15 +267,17 @@ private[clustering] trait LDAParams extends Params with HasFeaturesCol with HasM
    */
   @Since("1.6.0")
   final val optimizeDocConcentration = new BooleanParam(this, "optimizeDocConcentration",
-    "Indicates whether the docConcentration (Dirichlet parameter for document-topic" +
-      " distribution) will be optimized during training.")
+    "(For online optimizer only, currently) Indicates whether the docConcentration" +
+      " (Dirichlet parameter for document-topic distribution) will be optimized during training.")
 
   /** @group expertGetParam */
   @Since("1.6.0")
   def getOptimizeDocConcentration: Boolean = $(optimizeDocConcentration)
 
   /**
-   * For EM optimizer, if using checkpointing, this indicates whether to keep the last
+   * For EM optimizer only: [[optimizer]] = "em".
+   *
+   * If using checkpointing, this indicates whether to keep the last
    * checkpoint. If false, then the checkpoint will be deleted. Deleting the checkpoint can
    * cause failures if a data partition is lost, so set this bit with care.
    * Note that checkpoints will be cleaned up via reference counting, regardless.
@@ -279,7 +291,7 @@ private[clustering] trait LDAParams extends Params with HasFeaturesCol with HasM
    */
   @Since("2.0.0")
   final val keepLastCheckpoint = new BooleanParam(this, "keepLastCheckpoint",
-    "For EM optimizer, if using checkpointing, this indicates whether to keep the last" +
+    "(For EM optimizer) If using checkpointing, this indicates whether to keep the last" +
       " checkpoint. If false, then the checkpoint will be deleted. Deleting the checkpoint can" +
       " cause failures if a data partition is lost, so set this bit with care.")
 
diff --git a/mllib/src/main/scala/org/apache/spark/ml/regression/GBTRegressor.scala b/mllib/src/main/scala/org/apache/spark/ml/regression/GBTRegressor.scala
index cef7c643d7..8eb2984f7b 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/regression/GBTRegressor.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/regression/GBTRegressor.scala
@@ -41,6 +41,18 @@ import org.apache.spark.sql.functions._
  * [[http://en.wikipedia.org/wiki/Gradient_boosting Gradient-Boosted Trees (GBTs)]]
  * learning algorithm for regression.
  * It supports both continuous and categorical features.
+ *
+ * The implementation is based upon: J.H. Friedman. "Stochastic Gradient Boosting." 1999.
+ *
+ * Notes on Gradient Boosting vs. TreeBoost:
+ *  - This implementation is for Stochastic Gradient Boosting, not for TreeBoost.
+ *  - Both algorithms learn tree ensembles by minimizing loss functions.
+ *  - TreeBoost (Friedman, 1999) additionally modifies the outputs at tree leaf nodes
+ *    based on the loss function, whereas the original gradient boosting method does not.
+ *     - When the loss is SquaredError, these methods give the same result, but they could differ
+ *       for other loss functions.
+ *  - We expect to implement TreeBoost in the future:
+ *    [https://issues.apache.org/jira/browse/SPARK-4240]
  */
 @Since("1.4.0")
 @Experimental
diff --git a/mllib/src/main/scala/org/apache/spark/ml/regression/GeneralizedLinearRegression.scala b/mllib/src/main/scala/org/apache/spark/ml/regression/GeneralizedLinearRegression.scala
index a40d3731cb..05bf64591b 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/regression/GeneralizedLinearRegression.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/regression/GeneralizedLinearRegression.scala
@@ -165,7 +165,11 @@ class GeneralizedLinearRegression @Since("2.0.0") (@Since("2.0.0") override val
   setDefault(tol -> 1E-6)
 
   /**
-   * Sets the regularization parameter.
+   * Sets the regularization parameter for L2 regularization.
+   * The regularization term is
+   * {{{
+   *   0.5 * regParam * L2norm(coefficients)^2
+   * }}}
    * Default is 0.0.
    * @group setParam
    */
@@ -772,7 +776,7 @@ object GeneralizedLinearRegressionModel extends MLReadable[GeneralizedLinearRegr
  * :: Experimental ::
  * Summarizing Generalized Linear regression Fits.
  *
- * @param predictions predictions outputted by the model's `transform` method
+ * @param predictions predictions output by the model's `transform` method
  * @param predictionCol field in "predictions" which gives the prediction value of each instance
  * @param model the model that should be summarized
  * @param diagInvAtWA diagonal of matrix (A^T * W * A)^-1 in the last iteration
@@ -933,6 +937,9 @@ class GeneralizedLinearRegressionSummary private[regression] (
 
   /**
    * Standard error of estimated coefficients and intercept.
+   *
+   * If [[GeneralizedLinearRegression.fitIntercept]] is set to true,
+   * then the last element returned corresponds to the intercept.
    */
   @Since("2.0.0")
   lazy val coefficientStandardErrors: Array[Double] = {
@@ -941,6 +948,9 @@ class GeneralizedLinearRegressionSummary private[regression] (
 
   /**
    * T-statistic of estimated coefficients and intercept.
+   *
+   * If [[GeneralizedLinearRegression.fitIntercept]] is set to true,
+   * then the last element returned corresponds to the intercept.
    */
   @Since("2.0.0")
   lazy val tValues: Array[Double] = {
@@ -954,6 +964,9 @@ class GeneralizedLinearRegressionSummary private[regression] (
 
   /**
    * Two-sided p-value of estimated coefficients and intercept.
+   *
+   * If [[GeneralizedLinearRegression.fitIntercept]] is set to true,
+   * then the last element returned corresponds to the intercept.
    */
   @Since("2.0.0")
   lazy val pValues: Array[Double] = {
diff --git a/mllib/src/main/scala/org/apache/spark/ml/regression/LinearRegression.scala b/mllib/src/main/scala/org/apache/spark/ml/regression/LinearRegression.scala
index 9619e72a45..aacff4ea47 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/regression/LinearRegression.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/regression/LinearRegression.scala
@@ -513,7 +513,7 @@ object LinearRegressionModel extends MLReadable[LinearRegressionModel] {
  * Linear regression training results. Currently, the training summary ignores the
  * training weights except for the objective trace.
  *
- * @param predictions predictions outputted by the model's `transform` method.
+ * @param predictions predictions output by the model's `transform` method.
  * @param objectiveHistory objective function (scaled loss + regularization) at each iteration.
  */
 @Since("1.5.0")
@@ -549,7 +549,7 @@ class LinearRegressionTrainingSummary private[regression] (
  * :: Experimental ::
  * Linear regression results evaluated on a dataset.
  *
- * @param predictions predictions outputted by the model's `transform` method.
+ * @param predictions predictions output by the model's `transform` method.
  * @param predictionCol Field in "predictions" which gives the predicted value of the label at
  *                      each instance.
  * @param labelCol Field in "predictions" which gives the true label of each instance.
@@ -655,8 +655,11 @@ class LinearRegressionSummary private[regression] (
 
   /**
    * Standard error of estimated coefficients and intercept.
-   *
    * This value is only available when using the "normal" solver.
+   *
+   * If [[LinearRegression.fitIntercept]] is set to true,
+   * then the last element returned corresponds to the intercept.
+   *
    * @see [[LinearRegression.solver]]
    */
   lazy val coefficientStandardErrors: Array[Double] = {
@@ -679,8 +682,11 @@ class LinearRegressionSummary private[regression] (
 
   /**
    * T-statistic of estimated coefficients and intercept.
-   *
    * This value is only available when using the "normal" solver.
+   *
+   * If [[LinearRegression.fitIntercept]] is set to true,
+   * then the last element returned corresponds to the intercept.
+   *
    * @see [[LinearRegression.solver]]
    */
   lazy val tValues: Array[Double] = {
@@ -699,8 +705,11 @@ class LinearRegressionSummary private[regression] (
 
   /**
    * Two-sided p-value of estimated coefficients and intercept.
-   *
    * This value is only available when using the "normal" solver.
+   *
+   * If [[LinearRegression.fitIntercept]] is set to true,
+   * then the last element returned corresponds to the intercept.
+   *
    * @see [[LinearRegression.solver]]
    */
   lazy val pValues: Array[Double] = {
diff --git a/mllib/src/main/scala/org/apache/spark/ml/tree/impl/GradientBoostedTrees.scala b/mllib/src/main/scala/org/apache/spark/ml/tree/impl/GradientBoostedTrees.scala
index d365655674..b6334762c7 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/tree/impl/GradientBoostedTrees.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/tree/impl/GradientBoostedTrees.scala
@@ -30,22 +30,6 @@ import org.apache.spark.rdd.RDD
 import org.apache.spark.storage.StorageLevel
 
 
-/**
- * A package that implements
- * [[http://en.wikipedia.org/wiki/Gradient_boosting  Stochastic Gradient Boosting]]
- * for regression and binary classification.
- *
- * The implementation is based upon:
- *   J.H. Friedman.  "Stochastic Gradient Boosting."  1999.
- *
- * Notes on Gradient Boosting vs. TreeBoost:
- *  - This implementation is for Stochastic Gradient Boosting, not for TreeBoost.
- *  - Both algorithms learn tree ensembles by minimizing loss functions.
- *  - TreeBoost (Friedman, 1999) additionally modifies the outputs at tree leaf nodes
- *    based on the loss function, whereas the original gradient boosting method does not.
- *     - When the loss is SquaredError, these methods give the same result, but they could differ
- *       for other loss functions.
- */
 private[spark] object GradientBoostedTrees extends Logging {
 
   /**
diff --git a/python/pyspark/ml/classification.py b/python/pyspark/ml/classification.py
index d98919b3c6..e64c7a392b 100644
--- a/python/pyspark/ml/classification.py
+++ b/python/pyspark/ml/classification.py
@@ -291,7 +291,7 @@ class LogisticRegressionSummary(JavaCallable):
     @since("2.0.0")
     def probabilityCol(self):
         """
-        Field in "predictions" which gives the calibrated probability
+        Field in "predictions" which gives the probability
         of each class as a vector.
         """
         return self._call_java("probabilityCol")
diff --git a/python/pyspark/ml/regression.py b/python/pyspark/ml/regression.py
index f6c5d130dd..1c18df3b27 100644
--- a/python/pyspark/ml/regression.py
+++ b/python/pyspark/ml/regression.py
@@ -331,6 +331,9 @@ class LinearRegressionSummary(JavaCallable):
         Standard error of estimated coefficients and intercept.
         This value is only available when using the "normal" solver.
 
+        If :py:attr:`LinearRegression.fitIntercept` is set to True,
+        then the last element returned corresponds to the intercept.
+
         .. seealso:: :py:attr:`LinearRegression.solver`
         """
         return self._call_java("coefficientStandardErrors")
@@ -342,6 +345,9 @@ class LinearRegressionSummary(JavaCallable):
         T-statistic of estimated coefficients and intercept.
         This value is only available when using the "normal" solver.
 
+        If :py:attr:`LinearRegression.fitIntercept` is set to True,
+        then the last element returned corresponds to the intercept.
+
         .. seealso:: :py:attr:`LinearRegression.solver`
         """
         return self._call_java("tValues")
@@ -353,6 +359,9 @@ class LinearRegressionSummary(JavaCallable):
         Two-sided p-value of estimated coefficients and intercept.
         This value is only available when using the "normal" solver.
 
+        If :py:attr:`LinearRegression.fitIntercept` is set to True,
+        then the last element returned corresponds to the intercept.
+
         .. seealso:: :py:attr:`LinearRegression.solver`
         """
         return self._call_java("pValues")
diff --git a/python/pyspark/ml/tuning.py b/python/pyspark/ml/tuning.py
index da00f317b3..ea8c61b7ef 100644
--- a/python/pyspark/ml/tuning.py
+++ b/python/pyspark/ml/tuning.py
@@ -588,6 +588,8 @@ class TrainValidationSplit(Estimator, ValidatorParams, MLReadable, MLWritable):
 class TrainValidationSplitModel(Model, ValidatorParams, MLReadable, MLWritable):
     """
     Model from train validation split.
+
+    .. versionadded:: 2.0.0
     """
 
     def __init__(self, bestModel):
diff --git a/python/pyspark/ml/wrapper.py b/python/pyspark/ml/wrapper.py
index a2cf2296fb..bbeb6cfe6f 100644
--- a/python/pyspark/ml/wrapper.py
+++ b/python/pyspark/ml/wrapper.py
@@ -249,7 +249,7 @@ class JavaModel(Model, JavaCallable, JavaTransformer):
         """
         Initialize this instance with a Java model object.
         Subclasses should call this constructor, initialize params,
-        and then call _transformer_params_from_java.
+        and then call _transfer_params_from_java.
 
         This instance can be instantiated without specifying java_model,
         it will be assigned after that, but this scenario only used by
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/functions.scala b/sql/core/src/main/scala/org/apache/spark/sql/functions.scala
index 5bc0034cb0..223122300d 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/functions.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/functions.scala
@@ -154,6 +154,8 @@ object functions {
   /**
    * Aggregate function: returns the approximate number of distinct items in a group.
    *
+   * @param rsd maximum estimation error allowed (default = 0.05)
+   *
    * @group agg_funcs
    * @since 1.3.0
    */
@@ -164,6 +166,8 @@ object functions {
   /**
    * Aggregate function: returns the approximate number of distinct items in a group.
    *
+   * @param rsd maximum estimation error allowed (default = 0.05)
+   *
    * @group agg_funcs
    * @since 1.3.0
    */