[docs] [issue32216] Document PEP 557 Data Classes (dataclasses module)

[Ned Deily]

Ned Deily <nad at python.org> added the comment:

We really need to get this done prior to 370rc1 coming up on 05-21.

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priority: deferred blocker -> release blocker

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<https://bugs.python.org/issue32216>
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