Updated the comment for SPARK-2162.

A follow up on #1103

@andrewor14

Author: Reynold Xin <rxin@apache.org>

Closes #1117 from rxin/SPARK-2162 and squashes the following commits:

a4231de [Reynold Xin] Updated the comment for SPARK-2162.
Split to up to 3 parts.
Merge remote-tracking branch 'upstream/master' into SPARK-2177