Documentation: ABI: update sysfs-gpio to reference gpio-cdev

Update the sysfs-gpio interface document to refer to the gpio-cdev
interface that obsoletes it.
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Documentation/ABI/obsolete/sysfs-gpio

@@ -28,5 +28,5 @@ Description:
 	    /label ... (r/o) descriptive, not necessarily unique
 	    /ngpio ... (r/o) number of GPIOs; numbered N to N + (ngpio - 1)
 
-  This ABI is deprecated and will be removed after 2020. It is
-  replaced with the GPIO character device.
+  This ABI is obsoleted by Documentation/ABI/testing/gpio-cdev and will be
+  removed after 2020.