Add comments to USB examples

examples/HardwareTesting/PassThroughUSB/PassThroughUSB.ino

 #include <SD.h>
 #include <SerialFlash.h>
 
-AudioInputUSB            usb1;           //xy=200,69
+AudioInputUSB            usb1;           //xy=200,69  (must set Tools > USB Type to Audio)
 AudioOutputI2S           i2s1;           //xy=365,94
 AudioConnection          patchCord1(usb1, 0, i2s1, 0);
 AudioConnection          patchCord2(usb1, 1, i2s1, 1);

examples/HardwareTesting/WavFilePlayerUSB/WavFilePlayerUSB.ino

 #include <SerialFlash.h>
 
 AudioPlaySdWav           playWav1;
-AudioOutputUSB           audioOutput;
+AudioOutputUSB           audioOutput; // must set Tools > USB Type to Audio
 AudioOutputAnalog        dac;
 AudioConnection          patchCord1(playWav1, 0, audioOutput, 0);
 AudioConnection          patchCord2(playWav1, 1, audioOutput, 1);