api-ref: add a note about volume-backed rescue not being supported
api-ref: add a note about volume-backed rescue not being supported

This adds the same note for volume-backed instances not being
supported for the server rescue action as exists for the createBackup
action.

Change-Id: If4ba24f2fe15f479c353d0ea69bf0f1454540ef0

klmitch/nova,rahulunair/nova,klmitch/nova,openstack/nova,gooddata/openstack-nova,mahak/nova,mikalstill/nova,mahak/nova,mahak/nova,gooddata/openstack-nova,mikalstill/nova,phenoxim/nova,openstack/nova,phenoxim/nova,klmitch/nova,mikalstill/nova,rahulunair/nova,gooddata/openstack-nova,openstack/nova,klmitch/nova,rahulunair/nova,gooddata/openstack-nova