Documentation: ABI: add wideband filter type to sysfs-bus-iio

The Wideband Low Ripple filter is used for AD7768-1 Driver.
Document wideband filter option into filter_type_available
attribute.

Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/b390ec6d92dd742ace93bd8e40a0df4379b98e23.1741268122.git.Jonathan.Santos@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: jonathanns

Documentation/ABI/testing/sysfs-bus-iio

@@ -2290,6 +2290,8 @@ Description:
 		* "sinc3+pf2" - Sinc3 + device specific Post Filter 2.
 		* "sinc3+pf3" - Sinc3 + device specific Post Filter 3.
 		* "sinc3+pf4" - Sinc3 + device specific Post Filter 4.
+		* "wideband" - filter with wideband low ripple passband
+		  and sharp transition band.
 
 What:		/sys/bus/iio/devices/iio:deviceX/filter_type
 What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY-voltageZ_filter_type