The C++ engine source is relatively well documented using Doxygen comments.
Online API Docs
The C++ API documentation is available online at ezEngine.GitHub.io/api-docs. Note that this documentation is only updated for full releases.
Generating the Docs locally
If you need the latest up-to-date source documentation, you can easily generate the Doxygen documentation locally. A Doxyfile can be found under PathToEzRepository/Documentation/Doxyfile
We tried setting up an automated build for the documentation over at readthedocs.org, but were not happy with what we achieved. If you want to give it a try yourself, feel free to do so.