RTFM? How To Write a Manual Worth Reading

An anonymous reader writes with a link to Rich Bowen's insightful, detail laden piece at Opensource.com about improving documentation: Have you noticed that the more frequently a particular open source community tells you to RTFM, the worse the FM is likely to be? I've been contemplating this for years, and have concluded that this is because patience and empathy are the basis of good documentation, much as they are the basis for being a decent person. What's the best example you know of for open-source documentation? How about the worst?