Update alarm expression BNF documentation
Update alarm expression BNF documentation

Change-Id: I3afa5dd25ac9635bb0969a0ca3642cb6af4a3df4

openstack/monasca-api,hpcloud-mon/monasca-events-api,stackforge/monasca-api,oneilcin/monasca-events-api,hpcloud-mon/monasca-events-api,oneilcin/monasca-events-api,sapcc/monasca-api,openstack/monasca-api,openstack/monasca-api,stackforge/monasca-api,sapcc/monasca-api,stackforge/monasca-api,oneilcin/monasca-events-api,hpcloud-mon/monasca-events-api,hpcloud-mon/monasca-events-api,oneilcin/monasca-events-api,sapcc/monasca-api,stackforge/monasca-api