[SPARK-17017][ML][MLLIB][ML][DOC] Updated the ml/mllib feature selection docs for ChiSqSelector

## What changes were proposed in this pull request?

A follow up for #14597 to update feature selection docs about ChiSqSelector.

## How was this patch tested?

Generated html docs. It can be previewed at:

* ml: http://sparkdocs.lins05.pw/spark-17017/ml-features.html#chisqselector
* mllib: http://sparkdocs.lins05.pw/spark-17017/mllib-feature-extraction.html#chisqselector

Author: Shuai Lin <linshuai2012@gmail.com>

Closes #15236 from lins05/spark-17017-update-docs-for-chisq-selector-fpr.