media: vidioc-g-ext-ctrls.rst: document V4L2_CTRL_CLASS_DETECT

V4L2_CTRL_CLASS_DETECT was never documented here, add it.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/userspace-api/media/v4l/vidioc-g-ext-ctrls.rst

@@ -358,6 +358,10 @@ still cause this situation.
       - 0xa20000
       - The class containing RF tuner controls. These controls are
 	described in :ref:`rf-tuner-controls`.
+    * - ``V4L2_CTRL_CLASS_DETECT``
+      - 0xa30000
+      - The class containing motion or object detection controls. These controls
+        are described in :ref:`detect-controls`.
 
 Return Value
 ============