calibrate: Provide details about the output the touchscreen calibration is for

[Saviq]

Once we have a way to match touch HID devices to physical displays:

$ libinput.calibrate-touchscreen
Starting the calibrate-touchscreen app: touch the target spots.
(Or press <ESC> to quit!)
Output: Serial1234 Calibration = 1.008, 0.001, -0.002, -0.012, 1.003, 0.002
Output: Serial3245 Calibration = 1.003, 0.000, -0.005, -0.021, 1.007, 0.001

Together with #2, which would do all the work to match the touch panel together.