[SPARK-19402][DOCS] Support LaTex inline formula correctly and fix warnings in Scala/Java APIs generation

## What changes were proposed in this pull request?

This PR proposes three things as below:

- Support LaTex inline-formula, `\( ... \)` in Scala API documentation
  It seems currently,

  ```
  \( ... \)
  ```

  are rendered as they are, for example,

  <img width="345" alt="2017-01-30 10 01 13" src="https://cloud.githubusercontent.com/assets/6477701/22423960/ab37d54a-e737-11e6-9196-4f6229c0189c.png">

  It seems mistakenly more backslashes were added.

- Fix warnings Scaladoc/Javadoc generation
  This PR fixes t two types of warnings as below:

  ```
  [warn] .../spark/sql/catalyst/src/main/scala/org/apache/spark/sql/Row.scala:335: Could not find any member to link for "UnsupportedOperationException".
  [warn]   /**
  [warn]   ^
  ```

  ```
  [warn] .../spark/sql/core/src/main/scala/org/apache/spark/sql/internal/VariableSubstitution.scala:24: Variable var undefined in comment for class VariableSubstitution in class VariableSubstitution
  [warn]  * `${var}`, `${system:var}` and `${env:var}`.
  [warn]      ^
  ```

- Fix Javadoc8 break
  ```
  [error] .../spark/mllib/target/java/org/apache/spark/ml/PredictionModel.java:7: error: reference not found
  [error]  *                       E.g., {link VectorUDT} for vector features.
  [error]                                       ^
  [error] .../spark/mllib/target/java/org/apache/spark/ml/PredictorParams.java:12: error: reference not found
  [error]    *                          E.g., {link VectorUDT} for vector features.
  [error]                                            ^
  [error] .../spark/mllib/target/java/org/apache/spark/ml/Predictor.java:10: error: reference not found
  [error]  *                       E.g., {link VectorUDT} for vector features.
  [error]                                       ^
  [error] .../spark/sql/hive/target/java/org/apache/spark/sql/hive/HiveAnalysis.java:5: error: reference not found
  [error]  * Note that, this rule must be run after {link PreprocessTableInsertion}.
  [error]                                                  ^
  ```

## How was this patch tested?

Manually via `sbt unidoc` and `jeykil build`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #16741 from HyukjinKwon/warn-and-break.

2 files changed, 4 insertions, 4 deletions

diff --git a/mllib/src/main/scala/org/apache/spark/ml/feature/MinMaxScaler.scala b/mllib/src/main/scala/org/apache/spark/ml/feature/MinMaxScaler.scala
index 19978c97d2..f648deced5 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/feature/MinMaxScaler.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/feature/MinMaxScaler.scala
@@ -84,7 +84,7 @@ private[feature] trait MinMaxScalerParams extends Params with HasInputCol with H
  *    $$
  * </blockquote>
  *
- * For the case $E_{max} == E_{min}$, $Rescaled(e_i) = 0.5 * (max + min)$.
+ * For the case \(E_{max} == E_{min}\), \(Rescaled(e_i) = 0.5 * (max + min)\).
  *
  * @note Since zero values will probably be transformed to non-zero values, output of the
  * transformer will be DenseVector even for sparse input.
diff --git a/mllib/src/main/scala/org/apache/spark/ml/feature/package.scala b/mllib/src/main/scala/org/apache/spark/ml/feature/package.scala
index 5dd648aecc..d75a6dc937 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/feature/package.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/feature/package.scala
@@ -25,13 +25,13 @@ import org.apache.spark.sql.DataFrame
  *
  * The `ml.feature` package provides common feature transformers that help convert raw data or
  * features into more suitable forms for model fitting.
- * Most feature transformers are implemented as [[Transformer]]s, which transform one [[DataFrame]]
+ * Most feature transformers are implemented as [[Transformer]]s, which transform one `DataFrame`
  * into another, e.g., [[HashingTF]].
  * Some feature transformers are implemented as [[Estimator]]s, because the transformation requires
  * some aggregated information of the dataset, e.g., document frequencies in [[IDF]].
- * For those feature transformers, calling [[Estimator!.fit]] is required to obtain the model first,
+ * For those feature transformers, calling `Estimator.fit` is required to obtain the model first,
  * e.g., [[IDFModel]], in order to apply transformation.
- * The transformation is usually done by appending new columns to the input [[DataFrame]], so all
+ * The transformation is usually done by appending new columns to the input `DataFrame`, so all
  * input columns are carried over.
  *
  * We try to make each transformer minimal, so it becomes flexible to assemble feature