doc:  Add block device xlator details

Change-Id: I4c24a7f0a4339364a80e5d1be0fc8de6b9c189c1
BUG: 1086762
Signed-off-by: Humble Chirammal <hchiramm@redhat.com>
Reviewed-on: http://review.gluster.org/7809
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>