doc: iio: ad4695: update for calibration support

Calibration support has been added to the ad4695 driver, so update the
documentation to reflect this.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20240820-ad4695-gain-offset-v1-3-c8f6e3b47551@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: dlech

Documentation/iio/ad4695.rst

@@ -143,13 +143,18 @@ present, then the external reference voltage is used and the internal buffer is
 disabled. If ``refin-supply`` is present, then the internal buffered reference
 voltage is used.
 
+Gain/offset calibration
+-----------------------
+
+System calibration is supported using the channel gain and offset registers via
+the ``calibscale`` and ``calibbias`` attributes respectively.
+
 Unimplemented features
 ----------------------
 
 - Additional wiring modes
 - Threshold events
 - Oversampling
-- Gain/offset calibration
 - GPIO support
 - CRC support