bpo-45619: documentation of execution model: clarify and update binding summary (GH-29232)

This does two changes that are quite related.

0. it add that variable binding can occur in pattern matching, an update of 3.10
which seems to have been omitted from the list of bindings
1. Given how long the sentence already was, with even subcases in the middle of
the sentence, the commit breaks the sentence into an actual list.
(cherry picked from commit cd876c84932ecc2f7a6c41f3fc800a34d5b06b95)

Co-authored-by: Arthur Milchior <arthur@milchior.fr>