## 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.
c528812ce7
