[media] v4l2 framework doc: clarify locking

  high-latency devices.

Thanks to Hans de Goede for our discussions on this topic.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Thanks-to: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
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