gfapi: Add description of glfs_io_cbk function as a comment

No code change, just add a comment describing the callback function
type.

Change-Id: I3a5c5b3d3fe042b4f535963dd477806832c15704
BUG: 839950
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5696
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>