docs: iio: add precision about sampling_frequency_available

The documentation give some exemple on what format can be expected
from sampling_frequency_available sysfs attribute
Signed-off-by: Fabien Lahoudere <fabien.lahoudere@collabora.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio

@@ -61,8 +61,11 @@ What:		/sys/bus/iio/devices/triggerX/sampling_frequency_available
 KernelVersion:	2.6.35
 Contact:	linux-iio@vger.kernel.org
 Description:
-		When the internal sampling clock can only take a small
-		discrete set of values, this file lists those available.
+		When the internal sampling clock can only take a specific set of
+		frequencies, we can specify the available values with:
+		- a small discrete set of values like "0 2 4 6 8"
+		- a range with minimum, step and maximum frequencies like
+		  "[min step max]"
 
 What:		/sys/bus/iio/devices/iio:deviceX/oversampling_ratio
 KernelVersion:	2.6.38