To blow my own tiny trumpet for a moment, I've written and updated a manual to go with:
http://sourceforge.net/project... for each release.
However, it isn't terrific AND I worked as a technical author for a number of years, doing mainframe software manuals. This is my main point, good manuals [mine is not] are hard and probably require equivalent effort to the software itself. The other big obstacle is that in, for example, mainframe world there is formal review process, formalised customer feedback, errata etc. etc. Also, manuals are planned as a 'set' installation, operation, troubleshooting, API etc.
I don't know a lot of my customers and can only correct things that appear in the Google group. In my case, since it's a niche. there's not very much.
Actually there's an opportunity here as well, in that non-code people could also participate in their favourite projects by writing guides. Indeed sometimes they do, but not often enough and they're fragmentary.