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 <[e-mail]>
Signed-off-by: Stanimir Varbanov <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst

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     Cyclic intra macroblock refresh. This is the number of continuous
     macroblocks refreshed every frame. Each frame a successive set of
     macroblocks is refreshed until the cycle completes and starts from
-    the top of the frame. Applicable to H264, H263 and MPEG4 encoder.
+    the top of the frame. Setting this control to zero means that
+    macroblocks will not be refreshed.
+    Applicable to H264, H263 and MPEG4 encoder.
 
 ``V4L2_CID_MPEG_VIDEO_FRAME_RC_ENABLE (boolean)``
     Frame level rate control enable. If this control is disabled then