There is no document for this. The reason we didn't create such a document is different game pads and joysticks will map their buttons differently. This being said you can make your own document for you team. Plug in your joystick and open the FTC Controller Station and copy down which buttons light up as you press different buttons on the controller. Granted this doesn't let you map every single available button output from the Read Buttons VI (not all 12 are shown in the FTC Controller Station). To do this you will have to write a simple test VI to see what outputs correspond to each button on your game pad or joystick.
