[SQL] add @group tab in limit() and count()

group tab is missing for scaladoc

Author: Jacky Li <jacky.likun@gmail.com>

Closes #3458 from jackylk/patch-7 and squashes the following commits:

0121a70 [Jacky Li] add @group tab in limit() and count()

(cherry picked from commit bafee67ebad01f7aea2cd393a70b57eb8345eeb0)
Signed-off-by: Michael Armbrust <michael@databricks.com>