Terragen doesn't just need documentation, it needs video tutorials and recourses.
Who the hell reads huge pdf pages on how to create something, when a few simple youtube videos show it without confusion and wondering if you misinterpretated something.
That's what really bothers me about Terragen. It's one of the few 3D programs I've come across that doesn't have proper video tutorials. I learnt Zbrush and 3dsmax in only a few months, purely by video tuts and training dvd's. But learning Terragen is desperately combing google, this forum and hundreds of unclear documentation pages.
The only reason I chose Terragen over Vue was that I came across those basic tutorials, which got me started, but after that you're pretty much screwed
For zbrush or 3dsmax you can find video tutorials about every function on youtube, and limitless free recourses like base meshes etc. Try the same for Terragen...
Great program, but difficult to get started.