I'm building a public API in my current job and, a few weeks ago, I was scouting around for the good API design advice. Absolutely far and away the best resource that I found was this Google TechTalk by Joshua Bloch:
Parse's blog has a pretty good article on documenting APIs[0], with links to a number of nicely one API documentations (not dropbox's, but Github's, Stripe's, Backbone's, Mailgun's and their own)
http://www.youtube.com/watch?v=aAb7hSCtvGw
If you're interested in the topic and haven't seen it yet, I urge you to do so ASAP.