How to find out what to import?

I was always confused to find out what I need to import in order to use the function.

For example, when I use MFMailComposeViewController, I know from googling what I need

#import <MessageUI/MFMailComposeViewController.h>

and add: MessageUI.framework

From the help documentation, how do I know that I need these things?

http://developer.apple.com/library/ios/#documentation/MessageUI/Reference/MFMailComposeViewController_class/Reference/Reference.html

I want to know how best to use reference documents without relying on Google code codes to find out what I need to import in the future, so can someone explain how to understand this from the documentation?