[SPARK-15643][DOC][ML] Update spark.ml and spark.mllib migration guide from 1.6 to 2.0

## What changes were proposed in this pull request?
Update ```spark.ml``` and ```spark.mllib``` migration guide from 1.6 to 2.0.

## How was this patch tested?
Docs update, no tests.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #13378 from yanboliang/spark-13448.