Documentation: iio: ad4000: Add IIO Device characteristics section

Complement ad4000 IIO driver documentation with considerations about
``_scale_available`` attribute and table of typical channel attributes.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/6c8fe68d7d413ffdd33b154ba45600c7ce7013da.1743110188.git.marcelo.schmitt@analog.com


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