Check then make the MLlib Python evaluation and feature doc to be as complete as the Scala doc.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #6461 from yanboliang/spark-7918 and squashes the following commits:

940e3f1 [Yanbo Liang] truncate too long line and remove extra sparse
a80ae58 [Yanbo Liang] MLlib Python doc parity check for evaluation and feature