bpo-17140: Document multiprocessing's ThreadPool (GH-23812) (GH-23836)

Up until now, the `multiprocessing.pool.ThreadPool` class has gone
undocumented, despite being a public class in multiprocessing that is
included in `multiprocessing.pool.__all__`.
(cherry picked from commit 84ebcf271a2cc8bfd1762acb279502b8b6ef236e)

Co-authored-by: Matt Wozniski <mwozniski@bloomberg.net>