Add plug-in summary for osc doc
Add plug-in summary for osc doc

Stevedore Sphinx extension handles this comment.
http://docs.openstack.org/developer/python-openstackclient/plugin-commands.html

Change-Id: I19a61edd9a3c91f1dbd411e8436891552822acea

openstack/python-searchlightclient,openstack/python-searchlightclient