## 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.

(cherry picked from commit 2dc0d7ef)
Signed-off-by: Joseph K. Bradley <joseph@databricks.com>