iio: ABI: document "sinc4+rej60" filter_type

Add a bullet point for "sinc4+rej60" filter_type that is used in the
ad7124 driver.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: dlech

Documentation/ABI/testing/sysfs-bus-iio

@@ -2322,6 +2322,7 @@ Description:
 		* "sinc4+lp" - Sinc4 + Low Pass Filter.
 		* "sinc4+sinc1" - Sinc4 + averaging by 8. Low 1st conversion
 		  time.
+		* "sinc4+rej60" - Sinc4 + 60Hz rejection.
 		* "sinc5" - The digital sinc5 filter. Excellent noise
 		  performance
 		* "sinc5+avg" - Sinc5 + averaging by 4.