Fix doxygen documentation - doxygen was getting confused on the variable-length ATTR_NON_NULL_PTR_ARG() macro used on  function prototypes and was generating many return types for several functions in the documentation (thanks to David Lyons).

Descriptor processing routines now pass around void pointers rather than uint8_t pointers, as their destination datatype is know well known -- they are just streams of bytes until they are cast to the correct destination type by DESCRIPTOR_CAST.

Added new Printer Host mode ClassDriver demo.

Added table of supported classes and modes to the main USB Class Driver documentation.