[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, 5 insertions, 5 deletions

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/SQLImplicits.scala b/sql/core/src/main/scala/org/apache/spark/sql/SQLImplicits.scala
index 2caf723669..375df64d39 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/SQLImplicits.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/SQLImplicits.scala
@@ -259,7 +259,7 @@ abstract class SQLImplicits extends LowPrioritySQLImplicits {
  * Conflicting implicits are placed here to disambiguate resolution.
  *
  * Reasons for including specific implicits:
- * newProductEncoder - to disambiguate for [[List]]s which are both [[Seq]] and [[Product]]
+ * newProductEncoder - to disambiguate for `List`s which are both `Seq` and `Product`
  */
 trait LowPrioritySQLImplicits {
   /** @since 1.6.0 */
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/functions.scala b/sql/core/src/main/scala/org/apache/spark/sql/functions.scala
index 9a080fd3c9..cabe1f4563 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/functions.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/functions.scala
@@ -633,7 +633,7 @@ object functions {
   def skewness(columnName: String): Column = skewness(Column(columnName))
 
   /**
-   * Aggregate function: alias for [[stddev_samp]].
+   * Aggregate function: alias for `stddev_samp`.
    *
    * @group agg_funcs
    * @since 1.6.0
@@ -641,7 +641,7 @@ object functions {
   def stddev(e: Column): Column = withAggregateFunction { StddevSamp(e.expr) }
 
   /**
-   * Aggregate function: alias for [[stddev_samp]].
+   * Aggregate function: alias for `stddev_samp`.
    *
    * @group agg_funcs
    * @since 1.6.0
@@ -717,7 +717,7 @@ object functions {
   def sumDistinct(columnName: String): Column = sumDistinct(Column(columnName))
 
   /**
-   * Aggregate function: alias for [[var_samp]].
+   * Aggregate function: alias for `var_samp`.
    *
    * @group agg_funcs
    * @since 1.6.0
@@ -725,7 +725,7 @@ object functions {
   def variance(e: Column): Column = withAggregateFunction { VarianceSamp(e.expr) }
 
   /**
-   * Aggregate function: alias for [[var_samp]].
+   * Aggregate function: alias for `var_samp`.
    *
    * @group agg_funcs
    * @since 1.6.0