media: vidioc-subdev-g-client-cap.rst: document struct v4l2_subdev_client_capability

The struct v4l2_subdev_client_capability was never actually documented,
only the capability flags were documented.

Add this.

Signed-off-by: Hans Verkuil <[e-mail]>

Documentation/userspace-api/media/v4l/vidioc-subdev-g-client-cap.rst

···
 the kernel is older than the headers the userspace uses, and thus the capability
 is unknown to the kernel.
 
+.. tabularcolumns:: |p{1.5cm}|p{2.9cm}|p{12.9cm}|
+
+.. c:type:: v4l2_subdev_client_capability
+
+.. flat-table:: struct v4l2_subdev_client_capability
+    :header-rows:  0
+    :stub-columns: 0
+    :widths:       3 4 20
+
+    * - __u64
+      - ``capabilities``
+      - Sub-device client capabilities of the opened device.
+
+.. tabularcolumns:: |p{6.8cm}|p{2.4cm}|p{8.1cm}|
+
 .. flat-table:: Client Capabilities
     :header-rows:  1