If possible, document invariants as part of the code, not as a comment. Make it as hard as possible to misuse the thing. People not only don't read manuals, they also don't read comments. Which isn't to say you shouldn't comment your code.

I wouldn't use a comment in that situation either - I don't think a comment hurts, but I'd rather the code failed in an obvious way if I tried to use it incorrectly.

My comments are generally along the lines of "so you're here because you need to change something? here's what you need to know", rather than "this is how you use this thing".