[SPARK-3359][DOCS] Make javadoc8 working for unidoc/genjavadoc compatibility in Java API documentation

## What changes were proposed in this pull request?

This PR make `sbt unidoc` complete with Java 8.

This PR roughly includes several fixes as below:

- Fix unrecognisable class and method links in javadoc by changing it from `[[..]]` to `` `...` ``

  ```diff
  - * A column that will be computed based on the data in a [[DataFrame]].
  + * A column that will be computed based on the data in a `DataFrame`.
  ```

- Fix throws annotations so that they are recognisable in javadoc

- Fix URL links to `<a href="http..."></a>`.

  ```diff
  - * [[http://en.wikipedia.org/wiki/Decision_tree_learning Decision tree]] model for regression.
  + * <a href="http://en.wikipedia.org/wiki/Decision_tree_learning">
  + * Decision tree (Wikipedia)</a> model for regression.
  ```

  ```diff
  -   * see http://en.wikipedia.org/wiki/Receiver_operating_characteristic
  +   * see <a href="http://en.wikipedia.org/wiki/Receiver_operating_characteristic">
  +   * Receiver operating characteristic (Wikipedia)</a>
  ```

- Fix < to > to

  - `greater than`/`greater than or equal to` or `less than`/`less than or equal to` where applicable.

  - Wrap it with `{{{...}}}` to print them in javadoc or use `{code ...}` or `{literal ..}`. Please refer https://github.com/apache/spark/pull/16013#discussion_r89665558

- Fix `</p>` complaint

## How was this patch tested?

Manually 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)
```

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #16013 from HyukjinKwon/SPARK-3359-errors-more.

4 files changed, 4 insertions, 4 deletions

diff --git a/graphx/src/main/scala/org/apache/spark/graphx/GraphLoader.scala b/graphx/src/main/scala/org/apache/spark/graphx/GraphLoader.scala
index add21f41ea..f665727ef9 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/GraphLoader.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/GraphLoader.scala
@@ -32,7 +32,7 @@ object GraphLoader extends Logging {
    * id and a target id. Skips lines that begin with `#`.
    *
    * If desired the edges can be automatically oriented in the positive
-   * direction (source Id < target Id) by setting `canonicalOrientation` to
+   * direction (source Id is less than target Id) by setting `canonicalOrientation` to
    * true.
    *
    * @example Loads a file in the following format:
diff --git a/graphx/src/main/scala/org/apache/spark/graphx/impl/VertexPartitionBase.scala b/graphx/src/main/scala/org/apache/spark/graphx/impl/VertexPartitionBase.scala
index 8d608c99b1..8da46db98b 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/impl/VertexPartitionBase.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/impl/VertexPartitionBase.scala
@@ -57,7 +57,7 @@ private[graphx] object VertexPartitionBase {
  * concrete implementation. [[VertexPartitionBaseOps]] provides a variety of operations for
  * VertexPartitionBase and subclasses that provide implicit evidence of membership in the
  * `VertexPartitionBaseOpsConstructor` typeclass (for example,
- * [[VertexPartition.VertexPartitionOpsConstructor]]).
+ * `VertexPartition.VertexPartitionOpsConstructor`).
  */
 private[graphx] abstract class VertexPartitionBase[@specialized(Long, Int, Double) VD: ClassTag]
   extends Serializable {
diff --git a/graphx/src/main/scala/org/apache/spark/graphx/impl/VertexPartitionBaseOps.scala b/graphx/src/main/scala/org/apache/spark/graphx/impl/VertexPartitionBaseOps.scala
index 43594573cf..a8ed59b09b 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/impl/VertexPartitionBaseOps.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/impl/VertexPartitionBaseOps.scala
@@ -29,7 +29,7 @@ import org.apache.spark.util.collection.BitSet
 /**
  * A class containing additional operations for subclasses of VertexPartitionBase that provide
  * implicit evidence of membership in the `VertexPartitionBaseOpsConstructor` typeclass (for
- * example, [[VertexPartition.VertexPartitionOpsConstructor]]).
+ * example, `VertexPartition.VertexPartitionOpsConstructor`).
  */
 private[graphx] abstract class VertexPartitionBaseOps
     [VD: ClassTag, Self[X] <: VertexPartitionBase[X]: VertexPartitionBaseOpsConstructor]
diff --git a/graphx/src/main/scala/org/apache/spark/graphx/lib/TriangleCount.scala b/graphx/src/main/scala/org/apache/spark/graphx/lib/TriangleCount.scala
index 21b22968a1..2715137d19 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/lib/TriangleCount.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/lib/TriangleCount.scala
@@ -36,7 +36,7 @@ import org.apache.spark.graphx._
  * self cycles and canonicalizes the graph to ensure that the following conditions hold:
  * <ul>
  * <li> There are no self edges</li>
- * <li> All edges are oriented src &gt; dst</li>
+ * <li> All edges are oriented (src is greater than dst)</li>
  * <li> There are no duplicate edges</li>
  * </ul>
  * However, the canonicalization procedure is costly as it requires repartitioning the graph.