Added a documentation note saying AbortableTask will only work with the DB backend.
Added a documentation note saying AbortableTask will only work with the DB backend.

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