[docs] [issue37913] Document that __length_hint__ may return NotImplemented

[Steve Dower]

Change by Steve Dower <steve.dower at python.org>:


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pull_requests: +15507
pull_request: https://github.com/python/cpython/pull/15860

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue37913>
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