Lack of documentation. Extra actions creating

I don't know maybe I'm blind, or this project don't have a properly-made documentation at all. It more looks like a school project.
For example, I don't know how to extend standard create toolbar action, rewrite needed for me module's method. After researches in SOURCE codes, I saw generated methods for auto-handling, but hadn't seen any examples of custom action creation.
So, the question is, is there any normal documentation or so? This project seems to be huge and popular enough, you even provide "sponsor" version, but I don't see documentation for coders at all...