Doc: Add note for IPv6 port update special cases
Doc: Add note for IPv6 port update special cases

Change-Id: I85e17719eb4420a6cfd82ee54e60dd23ccfa0e95
Related-Bug: #1719806

mahak/neutron,openstack/neutron,mahak/neutron,openstack/neutron,mahak/neutron,openstack/neutron