Issue #19234: Documents socket.fileno() returns -1 on failure

Doc/library/socket.rst

@@ -927,14 +927,13 @@ to sockets.
 
 .. method:: socket.fileno()
 
-   Return the socket's file descriptor (a small integer).  This is useful with
-   :func:`select.select`.
+   Return the socket's file descriptor (a small integer), or -1 on failure. This
+   is useful with :func:`select.select`.
 
    Under Windows the small integer returned by this method cannot be used where a
    file descriptor can be used (such as :func:`os.fdopen`).  Unix does not have
    this limitation.
 
-
 .. method:: socket.get_inheritable()
 
    Get the :ref:`inheritable flag <fd_inheritance>` of the socket's file