media: docs: dev-decoder: add restrictions about CAPTURE buffers

CAPTURE buffers might be read by the hardware after they are dequeued,
which goes against the general idea that userspace has full control over
dequeued buffers. Explain why and document the restrictions that this
implies for userspace.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Acked-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>