[3.10] bpo-46769: Improve documentation for `typing.TypeVar` (GH-31712) (GH-31941)

* [3.10] bpo-46769: Improve documentation for `typing.TypeVar` (GH-31712)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
(cherry picked from commit 81b425d4dc43b60dd11a3e9abc5c84a4b8b384db)

* Remove references to `reveal_type`, add new section on `self` types