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

## 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**

  <img width="505" alt="2016-11-20 2 43 23" src="https://cloud.githubusercontent.com/assets/6477701/20457285/26f07e1c-aecb-11e6-9ae9-d9dee66845f4.png">

**After**
  <img width="499" alt="2016-11-20 1 27 17" src="https://cloud.githubusercontent.com/assets/6477701/20457240/409124e4-aeca-11e6-9a91-0ba514148b52.png">

## 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.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #15939 from HyukjinKwon/SPARK-3359-javadoc.

1 files changed, 13 insertions, 0 deletions

diff --git a/pom.xml b/pom.xml
index 024b2850d0..7c0b0b59dc 100644
--- a/pom.xml
+++ b/pom.xml
@@ -2478,10 +2478,23 @@
               <additionalparam>-Xdoclint:all -Xdoclint:-missing</additionalparam>
               <tags>
                 <tag>
+                  <name>example</name>
+                  <placement>a</placement>
+                  <head>Example:</head>
+                </tag>
+                <tag>
                   <name>note</name>
                   <placement>a</placement>
                   <head>Note:</head>
                 </tag>
+                <tag>
+                  <name>group</name>
+                  <placement>X</placement>
+                </tag>
+                <tag>
+                  <name>tparam</name>
+                  <placement>X</placement>
+                </tag>
               </tags>
             </configuration>
           </plugin>