doc: iio: Document intensity scale as poorly defined

Add comment about intensity scale being poorly defined and
having no proper units.

Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20231207134200.329174-1-marex@denx.de


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