For now there is no specific documentation for the joystick control (aside the code itself).
Some hints that may help you: for the joystick we use the ROS joy package which is a driver package to convert the joystick input into a well formatted ROS message. The node is started inside this ROS launch file.
As I speak the joystick control only works for Xbox controllers, and to control each joint separately. We might work on that in the future to also make the robot compatible with a Playstation controller, and (not sure yet) add a mode to control the end effector position instead of just the axis.