[docs] [issue17566] Document that importlib.abc.Loader.module_repr is abstract and thus needed by various other ABCs

[Brett Cannon]

Brett Cannon added the comment:

Barry, Eric: can you clarify why you made module_repr an abstractmethod and thus require its overloading? It seems like its default is fine and you should only need to overload it when you can say something better than the default.

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nosy: +barry, eric.smith

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