doc: add documentation for server-quorum feature

Change-Id: Ifaf5c18a320270a9b98c7b25536281fa42b64ad3
BUG: 1086765
Signed-off-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-on: http://review.gluster.org/7491
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>