Put enough into the code that it can stand alone. Nearly every project I've been in where the documentation was separate, it got out of date or wasn't done, partly that if it's a separate document it becomes a separate task, partly as management didn't allow for it as a task in budgetting. Documenting inline as part of the flow works much better in my experience.
Write the documentation in a form which most editors recognise is a block; for C++ this seems to be /* rather than //. That way you can fold it and just see the declarations.
