The API documentation is written in an XML format but translated into HTML. The most complete and up-to-date specification data is available here: http://cgit.freedesktop.org/NetworkManager/NetworkManager/tree/introspection
You can transform it into HTML with these steps:
checking out NM from git, http://www.gnome.org/projects/NetworkManager/developers/
- configure the NM build using --with-docs=yes, ie ./autogen.sh --with-docs=yes
- make docs/spec.html
Here is a snapshot of the spec for the 0.7.0 release from Nov 26 2008. (Could someone update it to the current release?)
Here is a DBus API snapshot for NM 0.7.996 from Nov 10 2009.