[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 6b52a58)

Co-authored-by: Sam Bull <[email protected]>

Author: miss-islington

Doc/library/asyncio-eventloop.rst

@@ -895,6 +895,9 @@ 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