#6591: add reference to ioctl in fcntl module for platforms other than Windows.

Doc/library/socket.rst

@@ -599,6 +599,9 @@ correspond to Unix system calls applicable to sockets.
    The :meth:`ioctl` method is a limited interface to the WSAIoctl system
    interface. Please refer to the MSDN documentation for more information.
 
+   On other platforms, the generic :func:`fcntl.fcntl` and :func:`fcntl.ioctl`
+   functions may be used; they accept a socket object as their first argument.
+
    .. versionadded:: 2.6