Add description to policies in used_limits
Add description to policies in used_limits

The 'discoverable' policy is not documented as it will be removed in a
future change.

blueprint policy-docs

Change-Id: I0ac7bc060d96d1f8a66044dc32e83ea747a4a4f6

vmturbo/nova,jianghuaw/nova,mikalstill/nova,rajalokan/nova,gooddata/openstack-nova,rahulunair/nova,vmturbo/nova,openstack/nova,klmitch/nova,vmturbo/nova,openstack/nova,klmitch/nova,mahak/nova,rajalokan/nova,Juniper/nova,Juniper/nova,mahak/nova,vmturbo/nova,rajalokan/nova,openstack/nova,phenoxim/nova,rahulunair/nova,rajalokan/nova,phenoxim/nova,Juniper/nova,klmitch/nova,gooddata/openstack-nova,Juniper/nova,jianghuaw/nova,mikalstill/nova,jianghuaw/nova,mahak/nova,gooddata/openstack-nova,rahulunair/nova,mikalstill/nova,gooddata/openstack-nova,klmitch/nova,jianghuaw/nova