[SPARK-18324][ML][DOC] Update ML programming and migration guide for 2.1 release

## What changes were proposed in this pull request?
Update ML programming and migration guide for 2.1 release.

## How was this patch tested?
Doc change, no test.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #16076 from yanboliang/spark-18324.