Update meaning for router:external
Update meaning for router:external

Excpilitly highlight that router:external is a flag that allow
floating IP creation on the network. Only this type of network
may be used as --external-gateway for routers. Reword statement
about external routing capabilities, as other provider networks
with router:external=False also might have external routing
capabilities.

Change-Id: I3d476680bb1ebf2024622cf734297e7825ef95ee

openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib