[Doc] Add note for port forwarding floating IP QoS
[Doc] Add note for port forwarding floating IP QoS

Now all floating IPs will have QoS functionality, so
adds the doc to users/operators.

Change-Id: I39c6bfe5c3ee5d0937c5b19deada0a8dcf1716df
Related-Bug: #1796925

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