If you need a global wiki you've got two options ( tree in fact)
* use mediawiki same way as moodle does, using it as http://docs.moodle.org
* create a course where all the users have access and set up a wiki (ewiki or nwiki ( I'd rather nwiki but I'm partial on this one ) and then just create links from the courses to the wiki pages as link resources.
* you can also create the wiki as an activity in the main course. (moodle home page)