iio: adc: ad7173: document sampling frequency behaviour

iio: adc: ad7173: document sampling frequency behaviour

The ADCs supported by this driver feature a sequencer that read in a
loop all the enabled chanels. When setting the individual sampling
frequency for each channel and enabling multiple channels, the effective
of each channel will be lower than the actual set value. Document this
behaviour in a comment.

Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Dumitru Ceclan <dumitru.ceclan@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20240607-ad4111-v7-8-97e3855900a0@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>