Documentation: ABI: added filter mode doc in sysfs-bus-iio

Documentation: ABI: added filter mode doc in sysfs-bus-iio

The filter mode / filter type property is used for ad4130
and ad7779 drivers, therefore the ABI doc file for ad4130
was removed, merging both of them in the sysfs-bus-iio.
Since one of the drivers is available from 6.1, the version
has been set to 6.1 for these attributes.

Signed-off-by: Ramona Alexandra Nechita <ramona.nechita@analog.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://patch.msgid.link/20241014143204.30195-3-ramona.nechita@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>