Improving doc for physical_device_mappings flag.
Improving doc for physical_device_mappings flag.

Since Mitaka a 1-M mapping is allowed between
a physnet and NICS.

Change-Id: Ie68e9b268420e7d61f44a10ed86746b9695becde
Closes-Bug: 1567904

AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals