Add documentation for NGram feature transformer.

Author: Feynman Liang <fliang@databricks.com>

Closes #7244 from feynmanliang/SPARK-8457 and squashes the following commits:

5aface9 [Feynman Liang] Pretty print Scala output and add API doc to each codetab
60d5ac0 [Feynman Liang] Inline API doc and fix indentation
736ccbc [Feynman Liang] NGram feature transformer documentation