iio: Documentation: add ABI documentation for in_proximity_sampling_frequency_available

Add entry for the in_proximity_sampling_frequency_available sysfs
entry to the existing sampling_frequency_available ABI documentation.
Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio

@@ -55,6 +55,7 @@ Description:
 		then it is to be found in the base device directory.
 
 What:		/sys/bus/iio/devices/iio:deviceX/sampling_frequency_available
+What:		/sys/bus/iio/devices/iio:deviceX/in_proximity_sampling_frequency_available
 What:		/sys/.../iio:deviceX/buffer/sampling_frequency_available
 What:		/sys/bus/iio/devices/triggerX/sampling_frequency_available
 KernelVersion:	2.6.35