[issue27689] Add documentation for typing.Generator

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

Michael, thank you for the patch. I have only one comment.
Generator is one of the few types in typing.py that behaves contravariantly. Maybe you could emphasize that it is contravariant in send type in your patch?

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nosy: +levkivskyi

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