These all require authors! :) Please consider contributing to our tutorial.
If you must use one of these widgets that are undocumented, I strongly suggest you take a look at their respective header files in the GTK distribution. GTK's function names are very descriptive. Once you have an understanding of how things work, it's not difficult to figure out how to use a widget simply by looking at its function declarations. This, along with a few examples from others' code, and it should be no problem.
When you do come to understand all the functions of a new undocumented widget, please consider writing a tutorial on it so others may benefit from your time.