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>