docs: iio: document the 'index' attribute too

Two out of three attributes are documented, document the third one too.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Link: https://lore.kernel.org/r/20210215143511.25471-4-luca@lucaceresoli.net


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