Added basic driver example use code to the library documentation.

Made the USARTStream global public and documented in the SerialStream module, allowing for the serial USART stream to be accessed via its handle rather than via the implicit stdout and stdin streams.

LUFA/Drivers/Peripheral/ADC.h

@@ -48,6 +48,9 @@
  *  Hardware ADC driver. This module provides an easy to use driver for the hardware
  *  ADC present on many AVR models, for the conversion of analogue signals into the
  *  digital domain.
+ *
+ *  \note The exact API for this driver may vary depending on the target used - see
+ *        individual target module documentation for the API specific to your target processor.
  */
 
 #ifndef __ADC_H__