Document rescue image optional parameter
Document rescue image optional parameter

Change-Id: I172ff7d3889f394b74f34ad5b891beee64fbb8b7

Akasurde/openstack-manuals,gongysh2004/openstack-manual,saeki-masaki/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,kairoaraujo/openstack-manuals,gongysh2004/openstack-manual,openstack/openstack-manuals,kairoaraujo/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,potsmaster/openstack-manuals,openstack/openstack-manuals,potsmaster/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,saeki-masaki/openstack-manuals,Akasurde/openstack-manuals,openstack/openstack-manuals