[3.14] gh-131421: Fix ASDL kw_defaults being `expr*` instead of `expr?*` (GH-133773) (GH-143269)

Also fix docs ASDL highlighting.
(cherry picked from commit f37f57dfe683163f390ef589301e4dd4608c4288)

Co-authored-by: Samuel <samuel@knutsen.co>