[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 8e75592)

Co-authored-by: Alex Waygood <[email protected]>

Author: miss-islington