clarify deferred fixed IP assignment for ports on routed networks
clarify deferred fixed IP assignment for ports on routed networks

This patch clarifies the routed network configuration doc with respect
to when a fixed IP is assigned to a port on a routed network. For
more details see related bug.

Change-Id: Id8e97afdfc488069ac3a910e017c34b085b4eb36
Closes-Bug: #1718320

huntxu/neutron,eayunstack/neutron,noironetworks/neutron,noironetworks/neutron,mahak/neutron,openstack/neutron,mahak/neutron,openstack/neutron,openstack/neutron,huntxu/neutron,eayunstack/neutron,mahak/neutron