Where to start with documentation?
IMHO, the most intelligent thing to do right now would be concentrating efforts on basic tutorials, since there are out of there plenty of potential Construct users that only waits to getting started with the program in the easiest way it's possible. So that the user base grows, and the word-by-mouth boosts.
Have you ever seen the tutorial games of TGF? Something like that would be perfect (manual - sample files).
Take a look at the manual from page 16: it shows, step by step, how to create a very simple game from zero, and in the meantime it explains the basics of the program every time you meet something new (an editor an object, ecc..). Just take a look to the first 4 tutorial games (fifth is useless), you'll understand clearly what I mean.
For my personal experience, step by step tutorials are a VERY concrete and effective way to learn such a tool from zero. Ages ago I learned quickly the basics of TGF while having no programming skills. Didn't need anything else.. I looked at the rest of the manual something like a couple of times in 5 years, afterwards.
Maybe today that we have broadband connections, in the form of video tutorials, if properly made, they could result even more immediate.