[3.10] gh-105993: Add possible None return type to asyncio.EventLoop.start_tls docs (GH-105995) (#106190)

(cherry picked from commit 6b52a58)

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

Author: miss-islington

Doc/library/asyncio-eventloop.rst

@@ -830,6 +830,9 @@ TLS Upgrade
    object only because the coder caches *protocol*-side data and sporadically
    exchanges extra TLS session packets with *transport*.
 
+   In some situations (e.g. when the passed transport is already closing) this
+   may return ``None``.
+
    Parameters:
 
    * *transport* and *protocol* instances that methods like