A few minor Spark SQL Scaladoc fixes.

Author: Reynold Xin <rxin@apache.org>

Closes #1139 from rxin/sparksqldoc and squashes the following commits:

c3049d8 [Reynold Xin] Fixed line length.
66dc72c [Reynold Xin] A few minor Spark SQL Scaladoc fixes.

(cherry picked from commit 5464e79175e2fc85e2cadf0dd7c9a45dad028326)
Signed-off-by: Reynold Xin <rxin@apache.org>