fmt traits docs should provide more context

[huonw]

Someone looking at some code with, say, #[derive(Debug)] might search the docs for Debug and land on Debug, which doesn't make it immediately obvious what it is for (it currently just says "Format trait for the :? format. Useful for debugging, all types should implement this."). It would be great if each trait had an example of using it to format something and maybe an explicit pointer to the main std::fmt docs.