[3.13] gh-74929: PEP 667 general docs update (gh-119291)

* 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)

(cherry picked from commit e870c852c0ea96fa4e4569e9c39c7ceb80ce858d)

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