doc: iobuf and iobref documentation.

Change-Id: Ib6d8121a968e8160c587a8553b7333262bb44975
BUG: 1118309
Signed-off-by: Poornima G <pgurusid@redhat.com>
Reviewed-on: http://review.gluster.org/8818
Reviewed-by: Prashanth Pai <ppai@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>