iio: consumer.h: Fix raw values documentation notes

The raw values notes mention 'ADC counts' and are not fully accurate.

Reword the notes in order to remove the 'ADC counts' and describe the
conversion needed between a raw value and a value in the standard units.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/r/20230623085830.749991-5-herve.codina@bootlin.com


Signed-off-by: Mark Brown <broonie@kernel.org>