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

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

Change-Id: I433b80ba880397c9322752240727ca60a2e2d53f
Closes-Bug: #1694298

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