Update doc for value-meta value
Update doc for value-meta value

Remove requirement that values can't be empty or null.

Change-Id: Ifb390779bc73c894d3091de3c7af830336cb11ab

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