I have used and contributed to some WIKIs and this is a very useful way to get new users the information required quickly. The new users become experts and the experts contribute to the WIKI.
The upside of a WIKI is that the user community helps create the documentation that they would like to have and transfer their experiences.
The downside to a WIKI is that as change occurs the WIKI could become out of date.
So a WIKI will help you generate the documentation, but you will still need to keep control of it somehow?