Add description to policies in server_external_events.py
Add description to policies in server_external_events.py

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

blueprint policy-docs

Change-Id: Ib4fd37bc962782e8bbea5de3cc88edf4114855c6

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