feature/gfid-access: Adding documentation

Adding documentation to feature 'gfid-access' translator.

Change-Id: Ib70c0957ea22580cef2c7219a31dd9df33e9cd37
BUG: 1086751
Signed-off-by: Kotresh H R <khiremat@redhat.com>
Reviewed-on: http://review.gluster.org/7862
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
Tested-by: Niels de Vos <ndevos@redhat.com>