gh-104232: Fix sys.settrace docs to reflect the reality

[gaogaotiantian]

In our current docs, we claim that:

The local trace function should return a reference to itself (or to another function for further tracing in that scope), or None to turn off tracing in that scope.

That is not the case, and for as far as we know.

Returning None in the local trace function has no effect and it's already the de facto. There are at least 3 issues about this (#104232, #78980 and #56201) and it should not be that difficult to fix this. Rather than changing the behavior which could potentially break someone's code, we can simply change our docs because we never achieved what we claimed anyway, and people are fine with it.

• Issue: Local trace function return value: bug or documentation issue #104232

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📚 Documentation preview 📚: https://cpython-previews--110516.org.readthedocs.build/

[brandtbucher]

Thanks for looking into it (this has always confused me too).

[miss-islington-app]

Thanks @gaogaotiantian for the PR, and @brandtbucher for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

[bedevere-app]

GH-111045 is a backport of this pull request to the 3.11 branch.

[miss-islington-app]

Thanks @gaogaotiantian for the PR, and @brandtbucher for merging it 🌮🎉.. I'm working now to backport this PR to: 3.12.
🐍🍒⛏🤖

[bedevere-app]

GH-111047 is a backport of this pull request to the 3.12 branch.