[3.12] Miscellaneous improvements to the typing docs (GH-105529) (#105567)

Miscellaneous improvements to the typing docs (GH-105529)

Mostly, these are changes so that we use shorter sentences and shorter paragraphs. In particular, I've tried to make the first sentence introducing each object in the typing API short and declarative.
(cherry picked from commit 8e755923c97d689ba7c7fe8deb50c1b169263264)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>