Add "default" behaviour to QoS policies documentation
Add "default" behaviour to QoS policies documentation

Added devref documentation related to the "default" behaviour
for QoS policies.

Change-Id: Ic3b149ec5dfdc732a4b5851237389abaef8992b7
Closes-Bug: #1694298

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