[3.13] gh-118888: Further PEP 667 docs updates (gh-119893)

[miss-islington]

• Clarify impact on default behaviour of exec, eval, etc
• Update documentation for changes to PyEval_GetLocals (Local variable assignment is broken when combined with threads + tracing + closures #74929)

Closes gh-11888
(cherry picked from commit 2180991)

Co-authored-by: Alyssa Coghlan ncoghlan@gmail.com

• Issue: Python 3.13.0b1: exec() does not populate locals() #118888

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📚 Documentation preview 📚: https://cpython-previews--119894.org.readthedocs.build/