First of all a personal heartfelt "thank you" to you for contributing towards open-source project. It's because of people like you, who rise above the personal benefits to a significant extent and contribute to the open/free source movement, the people like me from the third-world can hope to learn anything significant without fearing about the costs involved. So I do appreciate your concern about SO grabbing all the credit due out there, but I politely wish to point out the need of better documentation of various open/free projects:
The most important reason for me at least: The great RTFM famed man pages are the worst place to learn anything about how to use a particular tool for a newcomer because most of the times man pages lack good and varied examples.
To quote Einstein, "Example isn't another way to teach, it is the only way to teach." - Albert Einstein [1]
I have suffered endless time trying to learn things from man-pages. Spent time asking questions on forums until the expert people there saw the need to give some examples and offered help. I also thank them all.
SO documentation can add much needed value for the newcomers here as experts can add examples and newcomers can find them at one place.
This one is the greatest reason I support efforts like SO documentation project. Of course, I don't underplay the great work the open/free source contributors are doing. It may be the case that the great experts, may not find time to write down examples that a newcomer may need and the needs vary from one newcomer to another: One size doesn't fit all.
The distributed crowd-sourcing model nature of SO can tackle this problem in a much better way, unless the truly open efforts like archive.org or gnu.org come ahead with hosting solutions like this.
[1] http://www.alberteinsteinsite.com/quotes/einsteinquotes.html