## What changes were proposed in this pull request?

Currently, the docs for TF-IDF only refer to using HashingTF with IDF. However, CountVectorizer can also be used. We should probably amend the user guide and examples to show this.

## How was this patch tested?

unit tests and doc generation

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #12454 from hhbyyh/tfdoc.

mengxr https://issues.apache.org/jira/browse/SPARK-11289

I make some changes in ML feature extractors. I.e. TF-IDF, Word2Vec, and CountVectorizer. I add new example code in spark/examples, hope it is the right place to add those examples.

Author: Xusen Yin <yinxusen@gmail.com>

Closes #9266 from yinxusen/SPARK-11289.