Developer documentation for the Messaging Menu?
Try the libindicate-doc package (and then look at what it installed with dpkg -L libindicate-doc). That should contain what you need.
This isn't full documentation or anything, but it may be useful http://www.kryogenix.org/days/2011/01/16/working-with-the-ubuntu-messaging-menu
The Ubuntu Platform API reference is now online at the Ubuntu App Developer site.
Also you'll find more links to documentation on the resources section for the platform documentation there.