Document case in USB CDC code, fixes #467

teensy4/usb.c

 			usb_cdc3_line_rtsdtr = setup.wValue;
 		}
 		#endif
+		// fall through to next case, to always send ZLP ACK
 	  case 0x2321: // CDC_SEND_BREAK
 		endpoint0_receive(NULL, 0, 0);
 		return;