[MINOR] [MLLIB] [ML] [DOC] Minor doc fixes for StringIndexer and MetadataUtils

Changes:
* Make Scala doc for StringIndexerInverse clearer.  Also remove Scala doc from transformSchema, so that the doc is inherited.
* MetadataUtils.scala: “ Helper utilities for tree-based algorithms” —> not just trees anymore

CC: holdenk mengxr

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

Closes #8679 from jkbradley/doc-fixes-1.5.

2 files changed, 12 insertions, 21 deletions

diff --git a/mllib/src/main/scala/org/apache/spark/ml/feature/StringIndexer.scala b/mllib/src/main/scala/org/apache/spark/ml/feature/StringIndexer.scala
index b6482ffe0b..3a4ab9a857 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/feature/StringIndexer.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/feature/StringIndexer.scala
@@ -181,10 +181,10 @@ class StringIndexerModel (
 
 /**
  * :: Experimental ::
- * A [[Transformer]] that maps a column of string indices back to a new column of corresponding
- * string values using either the ML attributes of the input column, or if provided using the labels
- * supplied by the user.
- * All original columns are kept during transformation.
+ * A [[Transformer]] that maps a column of indices back to a new column of corresponding
+ * string values.
+ * The index-string mapping is either from the ML attributes of the input column,
+ * or from user-supplied labels (which take precedence over ML attributes).
  *
  * @see [[StringIndexer]] for converting strings into indices
  */
@@ -202,32 +202,23 @@ class IndexToString private[ml] (
   /** @group setParam */
   def setOutputCol(value: String): this.type = set(outputCol, value)
 
-  /**
-   * Optional labels to be provided by the user, if not supplied column
-   * metadata is read for labels. The default value is an empty array,
-   * but the empty array is ignored and column metadata used instead.
-   * @group setParam
-   */
+  /** @group setParam */
   def setLabels(value: Array[String]): this.type = set(labels, value)
 
   /**
-   * Param for array of labels.
-   * Optional labels to be provided by the user.
-   * Default: Empty array, in which case column metadata is used for labels.
+   * Optional param for array of labels specifying index-string mapping.
+   *
+   * Default: Empty array, in which case [[inputCol]] metadata is used for labels.
    * @group param
    */
   final val labels: StringArrayParam = new StringArrayParam(this, "labels",
-    "array of labels, if not provided metadata from inputCol is used instead.")
+    "Optional array of labels specifying index-string mapping." +
+      " If not provided or if empty, then metadata from inputCol is used instead.")
   setDefault(labels, Array.empty[String])
 
-  /**
-   * Optional labels to be provided by the user, if not supplied column
-   * metadata is read for labels.
-   * @group getParam
-   */
+  /** @group getParam */
   final def getLabels: Array[String] = $(labels)
 
-  /** Transform the schema for the inverse transformation */
   override def transformSchema(schema: StructType): StructType = {
     val inputColName = $(inputCol)
     val inputDataType = schema(inputColName).dataType
diff --git a/mllib/src/main/scala/org/apache/spark/ml/util/MetadataUtils.scala b/mllib/src/main/scala/org/apache/spark/ml/util/MetadataUtils.scala
index fcb517b5f7..96a38a3bde 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/util/MetadataUtils.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/util/MetadataUtils.scala
@@ -25,7 +25,7 @@ import org.apache.spark.sql.types.StructField
 
 
 /**
- * Helper utilities for tree-based algorithms
+ * Helper utilities for algorithms using ML metadata
  */
 private[spark] object MetadataUtils {