[SPARK-3249][DOC] Fix links in ScalaDoc that cause warning messages in `sbt/sbt unidoc`

## What changes were proposed in this pull request?

This PR proposes to fix ambiguous link warnings by simply making them as code blocks for both javadoc and scaladoc.

```
[warn] .../spark/core/src/main/scala/org/apache/spark/Accumulator.scala:20: The link target "SparkContext#accumulator" is ambiguous. Several members fit the target:
[warn] .../spark/mllib/src/main/scala/org/apache/spark/mllib/optimization/GradientDescent.scala:281: The link target "runMiniBatchSGD" is ambiguous. Several members fit the target:
[warn] .../spark/mllib/src/main/scala/org/apache/spark/mllib/fpm/AssociationRules.scala:83: The link target "run" is ambiguous. Several members fit the target:
...
```

This PR also fixes javadoc8 break as below:

```
[error] .../spark/sql/core/target/java/org/apache/spark/sql/LowPrioritySQLImplicits.java:7: error: reference not found
[error]  * newProductEncoder - to disambiguate for {link List}s which are both {link Seq} and {link Product}
[error]                                                   ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/LowPrioritySQLImplicits.java:7: error: reference not found
[error]  * newProductEncoder - to disambiguate for {link List}s which are both {link Seq} and {link Product}
[error]                                                                                ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/LowPrioritySQLImplicits.java:7: error: reference not found
[error]  * newProductEncoder - to disambiguate for {link List}s which are both {link Seq} and {link Product}
[error]                                                                                                ^
[info] 3 errors
```

## How was this patch tested?

Manually via `sbt unidoc > output.txt` and the checked it via `cat output.txt | grep ambiguous`

and `sbt unidoc | grep error`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #16604 from HyukjinKwon/SPARK-3249.

2 files changed, 3 insertions, 3 deletions

diff --git a/graphx/src/main/scala/org/apache/spark/graphx/Graph.scala b/graphx/src/main/scala/org/apache/spark/graphx/Graph.scala
index c55a5885ba..b3a3420b84 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/Graph.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/Graph.scala
@@ -331,7 +331,7 @@ abstract class Graph[VD: ClassTag, ED: ClassTag] protected () extends Serializab
 
   /**
    * Merges multiple edges between two vertices into a single edge. For correct results, the graph
-   * must have been partitioned using [[partitionBy]].
+   * must have been partitioned using `partitionBy`.
    *
    * @param merge the user-supplied commutative associative function to merge edge attributes
    *              for duplicate edges.
diff --git a/graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala b/graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala
index 90907300be..475bccf9bf 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala
@@ -428,7 +428,7 @@ class GraphOps[VD: ClassTag, ED: ClassTag](graph: Graph[VD, ED]) extends Seriali
    * Compute the connected component membership of each vertex and return a graph with the vertex
    * value containing the lowest vertex id in the connected component containing that vertex.
    *
-   * @see [[org.apache.spark.graphx.lib.ConnectedComponents$#run]]
+   * @see `org.apache.spark.graphx.lib.ConnectedComponents.run`
    */
   def connectedComponents(): Graph[VertexId, ED] = {
     ConnectedComponents.run(graph)
@@ -438,7 +438,7 @@ class GraphOps[VD: ClassTag, ED: ClassTag](graph: Graph[VD, ED]) extends Seriali
    * Compute the connected component membership of each vertex and return a graph with the vertex
    * value containing the lowest vertex id in the connected component containing that vertex.
    *
-   * @see [[org.apache.spark.graphx.lib.ConnectedComponents$#run]]
+   * @see `org.apache.spark.graphx.lib.ConnectedComponents.run`
    */
   def connectedComponents(maxIterations: Int): Graph[VertexId, ED] = {
     ConnectedComponents.run(graph, maxIterations)