NAME
        CMD_UPDATE -- Terminate the writing of a clip to the clipboard.

   FUNCTION
        Indicate to the clipboard that the previous write commands are
        complete and can be used for any pending pastes (reads).  This
        command cannot be issued while any of the write commands are
        pending.

   IO REQUEST
        io_Message      mn_ReplyPort set up
        io_Device       preset by opendevice
        io_Unit         preset by opendevice
        io_Command      CMD_UPDATE
        io_ClipID       the ClipID of the write

   RESULTS
        io_Error        non-zero if an error occurred