The software update notifications as defnined in the software update plug-in policies occur whenever a patching 'cycle' takes place, and a reboot is required. If you don't require a reboot, you can also prompt (or nag) the user to reboot.I like to do it on a schedule, say every 2-4 hours.
In any case, when you define the messages, you have to check the radio button to do so, and then type in your message. As Andrew said, it's limited in the character set, but it should be enough to get your point across.