[SPARK-13826][SQL] Revises Dataset ScalaDoc

[liancheng]

What changes were proposed in this pull request?

This PR revises Dataset API ScalaDoc. All public methods are divided into the following groups

• @groupname basic: Basic Dataset functions
• @groupname action: Actions
• @groupname untypedrel: Untyped Language Integrated Relational Queries
• @groupname typedrel: Typed Language Integrated Relational Queries
• @groupname func: Functional Transformations
• @groupname rdd: RDD Operations
• @groupname output: Output Operations

@since tag and sample code are also updated. We may want to add more sample code for typed APIs.

How was this patch tested?

Documentation change. Checked by building unidoc locally.

[liancheng]

cc @rxin @marmbrus @yhuai

[liancheng]

Aww... The diff is too large to be displayed, seems that we can't do inline commenting for this one :(

[SparkQA]

Test build #53340 has finished for PR 11769 at commit eae6641.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • * case class Book(title: String, words: String)
  • * case class Word(word: String)

[SparkQA]

Test build #53341 has finished for PR 11769 at commit 4fcf4e2.

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

[yhuai]

How about we break it to multiple PRs?

[liancheng]

The problem is that I reordered many methods so that those in the same group are defined together. Will revert these reordering changes. This can probably shrinks the diff down to acceptable size.

[SparkQA]

Test build #53407 has finished for PR 11769 at commit 6062f49.

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

[liancheng]

Well, already minimized changes, but still too large to display :(

[SparkQA]

Test build #53420 has finished for PR 11769 at commit 5b7279f.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • * case class Book(title: String, words: String)
  • * case class Word(word: String)

[SparkQA]

Test build #53427 has finished for PR 11769 at commit 50502a5.

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

[SparkQA]

Test build #53429 has finished for PR 11769 at commit 1c02a2e.

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

[rxin]

Thanks - going to merge this. I found some issues but I will just open a PR to fix them.