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/7492
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Lalatendu Mohanty <lmohanty@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>