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>
(cherry picked from commit 99afc640f27841ec819c0f32c01f567fd0b345f8)
Signed-off-by: Humble Chirammal <hchiramm@redhat.com>

Change-Id: I4ccbc234d0c88674e855d91529e286b24f1bed84
BUG: 1086762
Reviewed-on: http://review.gluster.org/7832
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Niels de Vos <ndevos@redhat.com>