doc: Make sysfs-bus-iio doc more exact

A few IIC channel descriptions explained used units as:
data is in foo "that can be processed into an" [unit] value. The "can be
processed into" is quite broad statement as it does not really explain
what this processing means. This makes units pretty much useless.

After discussion with Jonathan, it seems the units for these channels
should also be well-defined as for all other channels. The processing
means the standard scale and offset application that is used throughout
the IIO. Let's make it more obvious by stating that the units are [unit]
after scale ane offset are applied.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://lore.kernel.org/r/41eafb0caa510cddf650cf5ff940639a184f3005.1677331779.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>