iio: improve IIO_CONCENTRATION channel type description

IIO_CONCENTRATION together with INFO_RAW specifier is used for reporting
raw concentrations of pollutants. Raw value should be meaningless
before being properly scaled. Because of that description shouldn't
mention raw value unit whatsoever.

Fix this by rephrasing existing description so it follows conventions
used throughout IIO ABI docs.

Fixes: 8ff6b3bc ("iio: chemical: Add IIO_CONCENTRATION channel type")
Signed-off-by: Tomasz Duszynski <tomasz.duszynski@octakon.com>
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>