iio: documentation: Document scd4x calibration use

Add entries from Documentation/ABI/testing/sysfs-bus-iio-scd30
to Documentation/ABI/testing/sysfs-bus-iio. The attributes of the scd4x
and scd30 are common.

Remove Documentation/ABI/testing/sysfs-bus-iio-scd30.

Signed-off-by: Roan van Dijk <roan@protonic.nl>
Link: https://lore.kernel.org/r/20211008101706.755942-5-roan@protonic.nl


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