Explain how the NOTIFY feature of the properties work.

Feedback on the page titled: Qt 4.7.0: The Property SystemURL: http://doc.qt.nokia.com/4.7/properties.html
 
Message: This documentation does not explain in any way how the NOTIFY feature of the properties work. To figure it out, the user had to find samples using web search. It would have been helpful to find examples and specification here.