iio: ABI: docs: Document Senseair Sunrise ABI

Add documentation for the sysfs attributes of the sunrise_co2 driver.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Link: https://lore.kernel.org/r/20210920135413.140310-3-jacopo+renesas@jmondi.org


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