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>