Improve grant docs on when and how use domain arg
Improve grant docs on when and how use domain arg

Change-Id: I76184b229d48d8bd3268c803400c2ab9d3fcab17

dtroyer/python-openstacksdk,stackforge/python-openstacksdk,openstack/python-openstacksdk,openstack/python-openstacksdk,openstack-infra/shade,dtroyer/python-openstacksdk,openstack-infra/shade,stackforge/python-openstacksdk