doc for quota details extension
doc for quota details extension

This patch adds a blurb about the quota detail API extension to wrap
up the doc aspect of this feature.

Change-Id: I3de3a686d70d74e10d01880c1ec0f4dd5b047038
Closes-Bug: #1704914

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