Clarify description of neutron subnet-create
Clarify description of neutron subnet-create

Better explain the parameter of the neutron subnet-create command.

Change-Id: Ice519027ebeebbc343a10546748350fc6ed731ae
Closes-Bug: #1299833

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