More minor scaladoc cleanup for Spark SQL.

Author: Reynold Xin <rxin@apache.org>

Closes #1142 from rxin/sqlclean and squashes the following commits:

67a789e [Reynold Xin] More minor scaladoc cleanup for Spark SQL.

(cherry picked from commit 278ec8a203c7f1de2716d8284f9bdafa54eee1cb)
Signed-off-by: Reynold Xin <rxin@apache.org>