gh-74929: locals() documentation update for PEP 667

[ncoghlan]

Proposed documentation wording taken from the
withdrawn PEP 558 (which had switched to the
Python level semantics proposed in PEP 667 prior
to its withdrawal).

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📚 Documentation preview 📚: https://cpython-previews--118265.org.readthedocs.build/

• Issue: Local variable assignment is broken when combined with threads + tracing + closures #74929

[ncoghlan]

DO-NOT-MERGE label added, as this should only be merged after the PEP 667 implementation in #115153 has been completed and merged.

[nineteendo]

Wouldn't it better to convert the pull request back to a draft? GitHub automatically prevents you from merging those.

[ncoghlan]

Wouldn't it better to convert the pull request back to a draft? GitHub automatically prevents you from merging those.

Good point, done (enough years of experience where draft PRs weren't available that I still forget to use them when appropriate)

[ncoghlan]

I plan to eventually replace the latter two sentences here with something like "For details, refer to the 'Python 3.13 porting guide'" (with a link to the relevant part of the 3.13 What's New document). That will be in a later PR that adds an initial version of the What's New update for PEP 667, though.