[docs] [issue19270] Document that sched.cancel() doesn't distinguish equal events and can break order

[Antoine Pitrou]

Antoine Pitrou added the comment:

I'm not strong on a new invariant, however I think bug #1 would deserve fixing:

> 1. sched.cancel() can remove wrong event (because it uses equality instead identity).

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue19270>
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