Documentation: ABI: IIO: Add sinc5+avg to the filter_type_available list

Add the sinc5+avg filter type to the list of possible values for the
filter_type_available attribute.

The sinc5+avg filter type is handled by the ad4170 driver.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/7ae9ec6da3a3f0c33206880fcba35a17531cf219.1751895245.git.marcelo.schmitt@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: machschmitt

Documentation/ABI/testing/sysfs-bus-iio

@@ -2321,6 +2321,7 @@ Description:
 		* "sinc3+pf3" - Sinc3 + device specific Post Filter 3.
 		* "sinc3+pf4" - Sinc3 + device specific Post Filter 4.
 		* "sinc5+pf1" - Sinc5 + device specific Post Filter 1.
+		* "sinc5+avg" - Sinc5 + averaging by 4.
 		* "wideband" - filter with wideband low ripple passband
 		  and sharp transition band.