It's a method that Apple uses for its documentation, but I'm not sure it's "the help format" for iPhone... (I'm not sure that there *is* a help format for iPhone...)
I made the assumption the asker was wondering about the Apple help application. True, there is no standard format, but all the apps I've been involved in, and many I've seen, use HTML and a UIWebView to present documentation. PastryKit isn't as widely used, but can give you help an edge in UI conformance.