You can access a symbol's header file or documentation quickly from within the Xcode editor very quickly.

Open documentation for a symbol:
Say you have NSMutableArray in your code, and you want to see the header. Option-double-click on it and Xcode will bring up the documentation file.

Open header for a symbol:
Say you have NSMutableArray in your code, and you want to see the header. Command-double-click on NSMutableArray and Xcode will bring up the header file.