[3.12] gh-105993: Add possible `None` return type to `asyncio.EventLoop.start_tls` docs (GH-105995) (#106188)

gh-105993: Add possible `None` return type to `asyncio.EventLoop.start_tls` docs (GH-105995)
(cherry picked from commit 6b52a581c151914e59c8c367a03bc7309713a73b)

Co-authored-by: Sam Bull <git@sambull.org>