bpo-43938: improve dataclasses.FrozenInstanceError documentation

[Llandy3d]

Update documentation to reflect that dataclasses.FrozenInstanceError is derived by AttributeError.

https://bugs.python.org/issue43938

https://bugs.python.org/issue43938

[the-knights-who-say-ni]

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[ericvsmith]

This looks good, thanks. After you sign the CLA I'll merge it.

[miss-islington]

Thanks @Llandy3d for the PR, and @ericvsmith for merging it 🌮🎉.. I'm working now to backport this PR to: 3.8, 3.9.
🐍🍒⛏🤖

[bedevere-bot]

GH-25635 is a backport of this pull request to the 3.9 branch.

[bedevere-bot]

GH-25636 is a backport of this pull request to the 3.8 branch.