[SPARK-3359][BUILD][DOCS] Print examples and disable group and tparam tags in javadoc

[HyukjinKwon]

What changes were proposed in this pull request?

This PR proposes/fixes two things.

• Remove many errors to generate javadoc with Java8 from unrecognisable tags, @tparam and @group.

  [error] .../spark/mllib/target/java/org/apache/spark/ml/classification/Classifier.java:18: error: unknown tag: group
  [error]   /** @group setParam */
  [error]       ^
  [error] .../spark/mllib/target/java/org/apache/spark/ml/classification/Classifier.java:8: error: unknown tag: tparam
  [error]  * @tparam FeaturesType  Type of input features.  E.g., <code>Vector</code>
  [error]    ^
  ...
  

  It does not fully resolve the problem but remove many errors. It seems both @group and @tparam are unrecognisable in javadoc. It seems we can't print them pretty in javadoc in a way of @example here because they appear differently (both examples can be found in http://spark.apache.org/docs/2.0.2/api/scala/index.html#org.apache.spark.ml.classification.Classifier).

• Print @example in javadoc.
  Currently, there are few @example tag in several places.

  ./graphx/src/main/scala/org/apache/spark/graphx/Graph.scala:   * @example This operation might be used to evaluate a graph
  ./graphx/src/main/scala/org/apache/spark/graphx/Graph.scala:   * @example We might use this operation to change the vertex values
  ./graphx/src/main/scala/org/apache/spark/graphx/Graph.scala:   * @example This function might be used to initialize edge
  ./graphx/src/main/scala/org/apache/spark/graphx/Graph.scala:   * @example This function might be used to initialize edge
  ./graphx/src/main/scala/org/apache/spark/graphx/Graph.scala:   * @example This function might be used to initialize edge
  ./graphx/src/main/scala/org/apache/spark/graphx/Graph.scala:   * @example We can use this function to compute the in-degree of each
  ./graphx/src/main/scala/org/apache/spark/graphx/Graph.scala:   * @example This function is used to update the vertices with new values based on external data.
  ./graphx/src/main/scala/org/apache/spark/graphx/GraphLoader.scala:   * @example Loads a file in the following format:
  ./graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala:   * @example This function is used to update the vertices with new
  ./graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala:   * @example This function can be used to filter the graph based on some property, without
  ./graphx/src/main/scala/org/apache/spark/graphx/Pregel.scala: * @example We can use the Pregel abstraction to implement PageRank:
  ./graphx/src/main/scala/org/apache/spark/graphx/VertexRDD.scala: * @example Construct a `VertexRDD` from a plain RDD:
  ./repl/scala-2.10/src/main/scala/org/apache/spark/repl/SparkCommandLine.scala: * @example new SparkCommandLine(Nil).settings
  ./repl/scala-2.10/src/main/scala/org/apache/spark/repl/SparkIMain.scala:   * @example addImports("org.apache.spark.SparkContext")
  ./sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/LiteralGenerator.scala: * @example {{{
  

Before

After

How was this patch tested?

Maunally tested by jekyll build with Java 7 and 8

java version "1.7.0_80"
Java(TM) SE Runtime Environment (build 1.7.0_80-b15)
Java HotSpot(TM) 64-Bit Server VM (build 24.80-b11, mixed mode)

java version "1.8.0_45"
Java(TM) SE Runtime Environment (build 1.8.0_45-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.45-b02, mixed mode)

Note: this does not make sbt unidoc suceed with Java 8 yet but it reduces the number of errors with Java 8.

[HyukjinKwon]

This precedence respects scaladoc.

After this PR, javadoc looks like

[srowen]

Yeah, I hadn't seen this mechanism until you used it in the last PR. This is great, and probably the way we will finally get javadoc 8 to work. That's fantastic. Does this actually let sbt unidoc complete successfully on Java 8?

[SparkQA]

Test build #68895 has finished for PR 15939 at commit 03f88d4.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #68896 has finished for PR 15939 at commit 8514ec4.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

Oh, no. It seems still there are a lot of errors. I just meant to show what I have tried to test with .. :).
Thanks for approving!

[srowen]

Merged to master/2.1

[HyukjinKwon]

Thank you Sean!