doc : man page for snapshot commands

This patch also contains few modifications in admin documentation.

Change-Id: I7bc2a88e6cbcfe81dcfafc2956f5b7c5524b0f0b
BUG: 1122399
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/8357
Reviewed-by: Avra Sengupta <asengupt@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>