Fix typo in header comment of celery.canvas.

should is currently used instead of should not
Warn against using revoke+terminate programatically
Fixes typo in revoke command
Wording
Asynpool: Fixes race condition when child proces terminated fast
Fixes tests (requires kombu master)
Merge pull request #1613 from twright/patch-1

Fix typo in header comment of celery.canvas.