[3.13] gh-118888: Further PEP 667 docs updates (gh-119894)

* Clarify impact on default behaviour of exec, eval, etc
* Update documentation for changes to PyEval_GetLocals (gh-74929)

Closes gh-118888
(cherry picked from commit 2180991ea3d50f56595edae241cc92dd4e7de642)

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