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