GH-91635: clarify docs about closing of transport in asyncio (GH-98563)

(cherry picked from commit 2fdcc6f)

Co-authored-by: Kumar Aditya <[email protected]>

Author: miss-islington

Doc/library/asyncio-protocol.rst

@@ -156,7 +156,8 @@ Base Transport
    will be received.  After all buffered data is flushed, the
    protocol's :meth:`protocol.connection_lost()
    <BaseProtocol.connection_lost>` method will be called with
-   :const:`None` as its argument.
+   :const:`None` as its argument. The transport should not be
+   used once it is closed.
 
 .. method:: BaseTransport.is_closing()