gh-104219: Document that idunders can return NotImplemented

[Gouvernathor]

I tried to change as little as possible but a broader reformulation, or a separate paragraph explaining that when __iadd__ returns NotImplemented it's the same as if iadd didn't exist at all, could be deemed more readable.

• Issue: Inplace dunders can return NotImplemented, and that's not documented #104219

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📚 Documentation preview 📚: https://cpython-previews--104220.org.readthedocs.build/

[CAM-Gerlach]

I suggest using the relevant role so that NotImplemented is appropriately formatted and readers can easily jump to more information.

Additionally, you might want to add the role to the adjacent, otherwise-similar descriptions of the __add__ and __radd__ dunders as well (and perhaps explicitly mention for __add__ that the operation falls back to __radd__ if NotImplemented is returned?).

[Gouvernathor]

I did that, and in other places too (not in the NotImplemented section itself and only once in some paragraph where it comes up 3 or 4 times).
I think the __add__ explanation is clear enough as it is, since more explanations are given just afterwards, and even then I think it's not really the purpose of this PR.

[erlend-aasland]

Thanks for the report and PR!

[bedevere-app]

GH-116210 is a backport of this pull request to the 3.12 branch.

[bedevere-app]

GH-116211 is a backport of this pull request to the 3.11 branch.

[Gouvernathor]

Thank you !