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

[HyukjinKwon]

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.

[HyukjinKwon]

cc @srowen, could you take a look please?

[SparkQA]

Test build #71447 has finished for PR 16604 at commit 151d40b.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #71448 has finished for PR 16604 at commit 70c5e25.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[srowen]

Yes I've long observed that unidoc doesn't seem to allow certain links for some reason. Best to avoid them if they generate errors or warnings.

[HyukjinKwon]

@srowen, thank you for approving.

[srowen]

Merged to master