Add description to policies in security_groups.py
Add description to policies in security_groups.py

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

blueprint policy-docs

Change-Id: I1c7e539b195cc6bb81f0e4814560d6c3c9953694

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