Sharpen your skills
The current MSDN style .CHM help file is a great starting point for getting an idea about the wealth of the DNN API. There is a lot functionality in the DNN API that you can reuse: do not work harder, work smarter!
If you start clicking around and reading bits, you will stumble upon parts (methods, properties, etc.), that are not documented to the level that you expect or want.
Now it is time to really start learning!
Have a look at your unzipped source pack and find the .vb source file that contains the class, methods or property you are interested in. This is a great way to learn about the structure of the whole DNN solution as well.
Share your skills
DotNetNuke = Open Source = Community = Sharing.
if you understand this equation, you should be more than willing to share your learning process with documentation to all other 800.000+ registered members of the DNN community. And there is a structured way how to do this, well documented in a blog by Philip Beadle, have a look. Although this blog talks about DNN 5.4.3, the procedure it is not linked to any specific version of DNN. It takes a little time to set it up, but it works like charm.
The basics of this procedure is that you create a difference (patch) file with your additions to the code in such a way, that it can be implemented easily by DNN core developers. A great way to submit things to the DNN core, not only documentation but bug fixes and enhancements as well. Again, you have to understand the equation.
If you share your efforts I am sure that your contribution (you) will get a recognition from the community! When did you have chance to perform for an audience of 800.000+ people before?
If you are willing to learn a lot, and document your learning, why not become a volunteer for the DNN Reference team? There is no better way to have your learning experiences multiplied 800.000 fold. Just spend half an hour a month (more is allowed) to sharpen your skills and share your efforts.
Contact me at ernstpeter.tamminga AT dotnetnuke.com if you believe in the equation.