It is not enough to recruit experts (with or without pay) to write,
because they can't keep up with changes--I found that out working on
books about Linux at O'Reilly. But it is not enough to ask the
community to contribute micro-documents--that causes everything comes
out disorganized and of wildly varying quality.
My most comprehensive article on the issues is:
http://praxagora.com/andyo/professional/community_author_collaboration.html
Much more for the curious at:
http://www.praxagora.com/community_documentation/
At FLOSS Manuals (http://flossmanuals.net/), where I volunteer, we're
filling the gap with well-organized writing projects combining peer
review from the public with experts from various free software
packages. There's a very active mailing list and a lot of highly
praised output on the web site.
(I may go back to one of the articles cited in the posting and add
this comment to it.)