Documentation: ABI: IIO: Add filter_type documentation

A previous patch added documentation for filter_type_available attributes.
However, the description for the value attribute (filter_type) was missing.
Add documentation for filter_type sysfs ABI.

Fixes: 01bb12922b60 ("Documentation: ABI: added filter mode doc in sysfs-bus-iio")
Signed-off-by: Marcelo Schmitt <[e-mail]>
Reviewed-by: David Lechner <[e-mail]>
Link: https://patch.msgid.link/a8dbcca[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/ABI/testing/sysfs-bus-iio

···
 		* "sinc3+pf3" - Sinc3 + device specific Post Filter 3.
 		* "sinc3+pf4" - Sinc3 + device specific Post Filter 4.
 
+What:		/sys/bus/iio/devices/iio:deviceX/filter_type
+What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY-voltageZ_filter_type
+KernelVersion:	6.1
+Contact:	[e-mail]
+Description:
+		Specifies which filter type apply to the channel. The possible
+		values are given by the filter_type_available attribute.
+
 What:		/sys/.../events/in_proximity_thresh_either_runningperiod
 KernelVersion:	6.6
 Contact:	[e-mail]