doc: Add 'expose volume capabilities' details of bd-xlator

Change-Id: If7a89a36c2eca653e4fa7f59597ac8b03e4bc747
BUG: 1086749
Signed-off-by: Humble Chirammal <hchiramm@redhat.com>
Signed-off-by: Bharata B Rao <bharata.rao@gmail.com>
Reviewed-on: http://review.gluster.org/7844
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>