bpo-45554: Document multiprocessing.Process.exitcode values (GH-30142)

This addresses [bpo-45554]() by expanding the `exitcode` documentation to also describe what `exitcode` will be in cases of normal termination, `sys.exit()` called, and on uncaught exceptions.

Automerge-Triggered-By: GH:pitrou
(cherry picked from commit 3852269b91fcc8ee668cd876b3669eba6da5b1ac)

Co-authored-by: John Marshall <jmarshall@hey.com>