media: v4l2-ctrls.h: clarify the p_def argument of v4l2_ctrl_new_std_compound

It was not sufficiently clear how to create and use p_def.
Improve the documentation.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

include/media/v4l2-ctrls.h

@@ -685,7 +685,9 @@ struct v4l2_ctrl *v4l2_ctrl_new_std_menu_items(struct v4l2_ctrl_handler *hdl,
  * @p_def:     The control's default value.
  *
  * Sames as v4l2_ctrl_new_std(), but with support to compound controls, thanks
- * to the @p_def field.
+ * to the @p_def field. Use v4l2_ctrl_ptr_create() to create @p_def from a
+ * pointer. Use v4l2_ctrl_ptr_create(NULL) if the default value of the
+ * compound control should be all zeroes.
  *
  */
 struct v4l2_ctrl *v4l2_ctrl_new_std_compound(struct v4l2_ctrl_handler *hdl,