doc: Include details of thin volume creation in snapshot guide.

Change-Id: I12b49c0e8af2cc3d91c29300fdfc47d772d380d7
Signed-off-by: hchiramm <hchiramm@redhat.com>
Reviewed-on: http://review.gluster.org/8791
Reviewed-by: Rajesh Joseph <rjoseph@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>