[3.13] GH-109975: Copyedit 3.13 What's New: C API (GH-124313) (#124334)

* [3.13] GH-109975: Copyedit 3.13 What's New: C API (GH-124313)
(cherry picked from commit 9d0a75269c6ae361b1ed5910c3b3424ed93b6f6d)

* gh-118915: Add/fix docs entries for some new 3.13 C API (GH-124134)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Petr Viktorin <encukou@gmail.com>