>I do this because I feel that the project has something of value and can be improved

If you don't already feel the project has value, I doubt the availability of documentation would sway you.

>I shouldn't be forced to make a contribution to a project if I just don't think it's doing everything right.

You're not being forced to do anything. We're just saying different people have different priorities: what's important to you might not be important to other people. Who knows? Maybe the programmer thought the header file was sufficient. It might very well be more than enough for his own needs. Instead of saying "somebody should write detailed documentation", people expect that you will be that somebody, since you're the one who raised the issue.