glusterd: Added description for nfs.transport-type option in volume set help.

Change-Id: I9fe81dc1c3172158e8dd86c4fa2a04af18cb9dde
BUG: 782285
Signed-off-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-on: http://review.gluster.org/4582
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>