[issue40978] Document that typing.SupportsXXX protocols are runtime checkable

[Mariatta]

Mariatta <mariatta at python.org> added the comment:

>  because their runtime results are inconsistent with how Mypy handles them, producing both false positives and false negatives.

@guido or @ivan, do you have further thoughts or additional context to share about the above point?

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nosy: +Mariatta

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue40978>
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