iio: core: Enhance the kernel doc of modes and currentmodes iio_dev entries

Let's provide more details about these two variables because their
understanding may not be straightforward for someone not used to the IIO
subsystem internal logic. The different modes will soon be also be more
documented for the same reason.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20220207143840.707510-2-miquel.raynal@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>