doc: zerofill api documentation

Change-Id: I437735d68fdcc4a55f12fd51b1d40cd7a32ad783
BUG: 1086756
Signed-off-by: Humble Chirammal <hchiramm@redhat.com>
Reviewed-on: http://review.gluster.org/7800
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>
Tested-by: Anand Avati <avati@redhat.com>