iio: cdc: ad7746: Add device specific ABI documentation.

The datasheet description of offset calibration is complex, so for that
on just refer the reader to the device datasheet.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220626122938.582107-16-jic23@kernel.org

Documentation/ABI/testing/sysfs-bus-iio-cdc-ad7746

+What:		/sys/.../iio:deviceX/in_capacitableY_calibbias_calibration
+What:		/sys/.../iio:deviceX/in_capacitableY_calibscale_calibration
+KernelVersion:	6.1
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Write 1 to trigger a calibration of the calibbias or
+		calibscale. For calibscale, a full scale capacitance should
+		be connected to the capacitance input and a
+		calibscale_calibration then started.  For calibbias see
+		the device datasheet section on "capacitive system offset
+		calibration".