[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.