barrier: features.barrier should be a NO_DOC

features.barrier is turned on/off internally by snapshot feature and hence it
should not be exposed to the customer and this option should not be documented.

Change-Id: Id9a421f94e291f1dc77044904bb18dd730c2d43e
BUG: 1135691
Signed-off-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-on: http://review.gluster.org/8572
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>