media: Documentation: v42l_core: v4l2_ext_control

Describe p_area field from v4l2_ext_ctrl

Signed-off-by: Ricardo Ribalda Delgado <ribalda@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>