Staging: iio: Documented output / DAC interface

Added documentation for:
* /sys/bus/iio/devices/deviceX/outY_scale
* /sys/bus/iio/devices/deviceX/outY_raw
* /sys/bus/iio/devices/deviceX/outY&Z_raw

Signed-off-by: Roland Stigge <stigge@antcom.de>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>