V4L/DVB (13121): gspca - pac7311: add comment about JPEG header

Add comment about the meaning of the fixed JPEG header bytes used to
create each image.

Signed-off-by: Marton Nemeth <nm127@freemail.hu>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>