Documentation: ABI: testing: add max11410 doc

Adding documentation for Analog Devices max11410 adc userspace sysfs.
Signed-off-by: Ibrahim Tilki <Ibrahim.Tilki@analog.com>
Link: https://lore.kernel.org/r/20221003105903.229-4-Ibrahim.Tilki@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio-adc-max11410

+What:		/sys/bus/iio/devices/iio:deviceX/in_voltage_filterY_notch_en
+Date:		September 2022
+KernelVersion:  6.0
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Enable or disable a notch filter.
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_voltage_filterY_notch_center
+Date:		September 2022
+KernelVersion:  6.0
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Center frequency of the notch filter in Hz.