docs: iio: ad7944: add documentation for chain mode

Add documentation for chain mode support that was recently added to the
AD7944 ADC driver.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240425-iio-ad7944-chain-mode-v1-2-9d9220ff21e1@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>