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

[Steve Dower]

Steve Dower <steve.dower at python.org> added the comment:


New changeset a39a4c7439ed09069e619c10d30a6d0ec9332d1a by Steve Dower in branch 'master':
bpo-37913: Link to NotImplemented from new docs (GH-15860)
https://github.com/python/cpython/commit/a39a4c7439ed09069e619c10d30a6d0ec9332d1a


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