[SPARK-4932] Add help comments in Analytics

Trivial modifications for usability.

Author: Takeshi Yamamuro <linguin.m.s@gmail.com>

Closes #3775 from maropu/AddHelpCommentInAnalytics and squashes the following commits:

fbea8f5 [Takeshi Yamamuro] Add help comments in Analytics

(cherry picked from commit 9c251c555f5ee527143d0cdb9e6c3cb7530fc8f8)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>