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

[STINNER Victor]

STINNER Victor added the comment:

@Raymond: The logic is essentially the same as is used in popular event loops like Tornado.

Sorry, I didn't understand your sentence: do you mean that sched must keep the insertion order or the order is undefined?

Undefined order is fine if it's documented.

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<http://bugs.python.org/issue19270>
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