gh-74929: PEP 667 general docs update

[ncoghlan]

• expand on What's New entry for PEP 667 (including porting notes)
• review (and update) all mentions of locals and f_locals in documentation
• define "optimised scope" as a new glossary term

C API documentation updates will be added in a separate PR

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

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📚 Documentation preview 📚: https://cpython-previews--119201.org.readthedocs.build/

[ncoghlan]

In addition to the inline notes in the PR, I also need to search for references to f_locals to see if any of those need changing.

[terryjreedy]

Does the following refer to something other than the c-api/frame.rst changes?

C API documentation updates will be added in a separate PR

[terryjreedy]

I did not see any grammar or spelling issues.

[jeremyhylton]

I left a handful of comments. Overall looks very good, but suggest a few minor revisions for clarity.

[ncoghlan]

Does the following refer to something other than the c-api/frame.rst changes?

There are several C API additions and updates as described in the PEP. Most of them will be covered in frame.rst, but there are a few PyEval_* changes to be covered elsewhere.

[ncoghlan]

I realised that I was using the Aus/UK spelling of optimised, while the Python docs generally use US spelling (there were 300+ instances of optimized vs less than 20 uses of optimised, including the additions in this PR).

I also added an inline note to the glossary entry pointing out that most optimizations are applied to all scopes, there's just a subset that are only valid when the local and nonlocal variables names are known at compile time.

To avoid more conflicts with sprint changes, I will merge this version once CI is happy.

[ncoghlan]

Capturing the commit message, just in case something fails before the auto-merge lands:

* expand on What's New entry for PEP 667 (including porting notes) * define 'optimized scope' as a glossary term * cover comprehensions and generator expressions in locals() docs * review all mentions of "locals" in documentation (updating if needed) * review all mentions of "f_locals" in documentation (updating if needed) 

[miss-islington-app]

Thanks @ncoghlan for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

[bedevere-app]

GH-119291 is a backport of this pull request to the 3.13 branch.