## 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.