iio: ABI: Document in_concentration_co2_scale

Document the 'in_concentration_co2_scale' standard IIO attribute.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Link: https://lore.kernel.org/r/20210920135413.140310-5-jacopo+renesas@jmondi.org


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