Document that return values of the task handlers (on_success, on_failure, on_retry) is ignored.
Document that return values of the task handlers (on_success, on_failure,
on_retry) is ignored.

cbrepo/celery,cbrepo/celery,ask/celery,mitsuhiko/celery,WoLpH/celery,ask/celery,WoLpH/celery,frac/celery,mitsuhiko/celery,frac/celery