Make the mountpoint property description clearer
Make the mountpoint property description clearer

There is an issue where the mountpoint may not be honored
so add this note to the property documentation so users can be
aware of the issue.

Change-Id: Iddab9b1f6e5fadddd00e651e92b8b9c922a59347
Closes-bug: #1231705

jasondunsmore/heat,cryptickp/heat,jasondunsmore/heat,pshchelo/heat,dragorosson/heat,gonzolino/heat,rh-s/heat,gonzolino/heat,rdo-management/heat,cryptickp/heat,NeCTAR-RC/heat,maestro-hybrid-cloud/heat,srznew/heat,dims/heat,pshchelo/heat,openstack/heat,rh-s/heat,redhat-openstack/heat,noironetworks/heat,pratikmallya/heat,srznew/heat,dragorosson/heat,NeCTAR-RC/heat,rdo-management/heat,steveb/heat,maestro-hybrid-cloud/heat,noironetworks/heat,steveb/heat,miguelgrinberg/heat,ntt-sic/heat,takeshineshiro/heat,ntt-sic/heat,cwolferh/heat-scratch,pratikmallya/heat,openstack/heat,dims/heat,cwolferh/heat-scratch,redhat-openstack/heat,miguelgrinberg/heat,takeshineshiro/heat