doc: Move bd translator documentation to xlators directory.

Change-Id: I21bd1cc6d0220035d28ca15e1e32b25a6c5a184a
Signed-off-by: hchiramm <hchiramm@redhat.com>
Reviewed-on: http://review.gluster.org/8700
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>