[SPARK-13826][SQL] Revises Dataset ScalaDoc

## 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.

Author: Cheng Lian <lian@databricks.com>

Closes #11769 from liancheng/spark-13826-ds-api-doc.