media: v4l2-compat-ioctl32.c: add missing documentation for a field

Fix this sparse warning:

drivers/media/v4l2-core/v4l2-compat-ioctl32.c:256: warning:
Function parameter or member 'capabilities' not described in 'v4l2_create_buffers32'
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/v4l2-core/v4l2-compat-ioctl32.c

@@ -244,6 +244,7 @@ struct v4l2_format32 {
  *		return: number of created buffers
  * @memory:	buffer memory type
  * @format:	frame format, for which buffers are requested
+ * @capabilities: capabilities of this buffer type.
  * @reserved:	future extensions
  */
 struct v4l2_create_buffers32 {