@futurist : True, most opensource projects favor wikis to document their software. The "wiki option" has been discussed a lot inside the team. There are those who are for it and those against it. So far, we have used MODx powered documentation, and it’s not (yet) opened to public contributions. But openning access to would be contributors would not be harder with MODx than with a wiki. The only thing you’d lack is revisionning (at least for now

).
From my experience running the french doc @TextBook (the Textpattern doc), wiki have pros and cons. The main con that you hear from users is learning a new syntax, which, to write technical doc can be more annoying that relatively simple text (inserting images, with MediaWiki, was not an easy process if you wanted those hosted on the documentation’s server... you had to have a FTP access on top of wiki privileges...). Back at Textpattern, people would have favored Textile since it’s native syntax to edit documents. I had suggested Instiki or DocuWiki, who can both use Textile or Markdown.
Also, opening contributions to the public means you have enough people who regularly fix vandalized pages, but also you have to have several "editors" so to speak to ensure coherence and smoothen out inexperienced writers personnal styles... I have done it for a year, it’s major work if you want a consistent and quality documentation.
My personnal take on this is : keep MODx as the documentation tool, and open documentation to people ready to get involved, but not make documentation editable by anyone. We don’t have enough resources to handle this...
IMHO, the tool is important in the sense that it does not get in the way of people’s involvement and makes their life easier, but it’s not so much the tool that matters - rather the way we choose to use it.