This is a question that have somewhat been asked before (i.e.: How to manage an open source project's documentation). However, my question is a little different because:

• We're not developing open source software, but proprietary software
• The documentation has to be hand-written, because we do not want to publish the actual software API documentation, but only the public API documentation
• I do want developers and project managers to write the documentation collaboratively

Obviously, wikis are a solution, but they're very generic. I'm looking for a more specialized tool for this job. I've looked around and found a few like Adobe Robohelp, SaaS solutions and such, but I'd like to know if any open source software exists for that purpose.

Do you know any Open Source Web-based CMS for writing and managing API and software documentation?