Summary

I couldn't get Sphinx to generate docstrings for@cached_propertys on attrs' defaultslots=True classes. Trying to replace the generated__getattr__ didn't perform so well...but,__slots__ can be a dictionary, and that dictionary can have docstrings in it. This PR makes attrs generate__slots__ as a dictionary, with docstrings copied from the decorated@cached_property functions.

I haven't worked out how to make Sphinx render these like@property. Might be doable.

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