Documentation/ABI/testing: Add documentation for AD5766 new ABI

New interface is proposed for dither functionality. This future allows
composing  an external signals to the selected output channel.
The dither signal can be turned on/off, scaled, inverted, or it can be
selected from different sources.
Signed-off-by: Cristian Pop <cristian.pop@analog.com>
Link: https://lore.kernel.org/r/20210115112105.58652-2-cristian.pop@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio-dac-ad5766

+What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY_dither_enable
+KernelVersion:	5.12
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Dither enable. Write 1 to enable dither or 0 to disable it.
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY_dither_invert
+KernelVersion:	5.12
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Inverts the dither applied to the selected DAC channel. Dither is not
+		inverted by default. Write "1" to invert dither.
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY_dither_scale_available
+KernelVersion:	5.12
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Returns possible scalings available for the current channel.
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY_dither_scale
+KernelVersion:	5.12
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Scales the dither before it is applied to the selected channel.
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY_dither_source
+KernelVersion:	5.12
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Selects dither source applied to the selected channel. Write "0" to
+		select N0 source, write "1" to select N1 source.