"Telling somebody to "look it up in the Wiki" is tantamount to telling them to go f* themselves." Ha! So very true.
Putting up a Wiki as documentation for a project/API, essentially means "we don't have time to write documentation, so we jotted together some napkin notes, maybe you can come in and fix them".
I've never seen "wiki for documentation" work in practice. It's always left in some semi-incoherent state, stale and incomplete.