media: docs: ext-ctrls-codec: Document cyclic intra-refresh zero control value

In all drivers _CYCLIC_INTRA_REFRESH_MB default control value is zero
which means that the macroblocks will not be intra-refreshed. Document
this _CYCLIC_INTRA_REFRESH_MB control behaviour in control description.

Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>