doc: Document libgfapi and its apis,usage,bindings..etc

Change-Id: I3435f9a88acafa905e879c68bb7aa8b16023cf02
BUG: 1086766
Signed-off-by: Humble Chirammal <hchiramm@redhat.com>
Signed-off-by: Soumya k <skoduri@redhat.com>
Reviewed-on: http://review.gluster.org/7865
Reviewed-by: Lalatendu Mohanty <lmohanty@redhat.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Niels de Vos <ndevos@redhat.com>