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>