Ask HN: Have you ever written docs for an open source project you're new to?

So I've occasionally, upon trying to join an open source project, been told that the best way to get started was to write documentation for it. In fact, that if you're new you are a great person to write documentation because you know what is confusing. I've been programming for about 8 years now and thought I would be able to do this by the time I graduated uni or had a few years in industry. That's happened and I've never actually been able to do this successfully and always ended up walking away in frustration. Has anyone ever written good documentation for an unfamiliar project? How do you go about quickly getting an understanding of the conceptual structure of the project from a high level?