Documentation comments are shown inside Xcode’s Quick Help, its code completion, and other places. To add one, press ⌥⌘/ then type in the comment you want to use.
Note: Xcode sometimes refuses to make this shortcut work. If you go to the Editor menu, choose Structure, and see that Add Documentation is permanently grayed out, try quitting Xcode, renaming it to _Xcode, then renaming it *back* to Xcode. Yes, really.
