[SPARK-20505][ML] Add docs and examples for ml.stat.Correlation and ml.stat.ChiSquareTest.

## What changes were proposed in this pull request?
Add docs and examples for ```ml.stat.Correlation``` and ```ml.stat.ChiSquareTest```.

## How was this patch tested?
Generate docs and run examples manually, successfully.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #17994 from yanboliang/spark-20505.