Update kvs assignment backend docs
Update kvs assignment backend docs

The "key to data" mapping was not documented for the KVS
assignment backend.

Change-Id: Id3cf3e0d9b52ceae5099e6d3833cc88f7aa0f347

openstack/keystone,jamielennox/keystone,jumpstarter-io/keystone,rodrigods/keystone,rajalokan/keystone,ilay09/keystone,dstanek/keystone,maestro-hybrid-cloud/keystone,takeshineshiro/keystone,openstack/keystone,reeshupatel/demo,dstanek/keystone,JioCloud/keystone,jonnary/keystone,cernops/keystone,idjaw/keystone,openstack/keystone,blueboxgroup/keystone,idjaw/keystone,mahak/keystone,roopali8/keystone,ging/keystone,ajayaa/keystone,promptworks/keystone,JioCloud/keystone,jumpstarter-io/keystone,promptworks/keystone,ajayaa/keystone,jumpstarter-io/keystone,nuxeh/keystone,jamielennox/keystone,rushiagr/keystone,rushiagr/keystone,himanshu-setia/keystone,rajalokan/keystone,rushiagr/keystone,jonnary/keystone,vivekdhayaal/keystone,promptworks/keystone,UTSA-ICS/keystone-kerberos,klmitch/keystone,vivekdhayaal/keystone,nuxeh/keystone,MaheshIBM/keystone,rodrigods/keystone,roopali8/keystone,reeshupatel/demo,cernops/keystone,ging/keystone,dstanek/keystone,reeshupatel/demo,maestro-hybrid-cloud/keystone,vivekdhayaal/keystone,takeshineshiro/keystone,rajalokan/keystone,dims/keystone,blueboxgroup/keystone,mahak/keystone,UTSA-ICS/keystone-kerberos,klmitch/keystone,dims/keystone,MaheshIBM/keystone,ilay09/keystone,himanshu-setia/keystone,mahak/keystone,ilay09/keystone,nuxeh/keystone