iio: imu: st_lsm6dsx: improve kernel docs and comments

Improve kernel docs and comments reporting supported sensors in a list in
order to make more easy to add new devices in the future.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/d365d048253d2ff9ee0092d391bbaa9cf0737cff.1706961432.git.lorenzo@kernel.org


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