Callback Framework for C++
The documentation is not available online. You can download a tarball from GitHub releases (since 2.10.1) or the GNOME download site (until 2.12.1), extract it, and view the documentation at untracked/docs/.
The latest tutorial-style and reference documentation is available on the web.
For those which just want a quick look at the terms used in the system, glance over the glossary