media: v4l2: clarify H.264 loop filter offset controls

The venus and s5p-mfc drivers add the loop filter alpha/beta offset
controls V4L2_CID_MPEG_VIDEO_H264_LOOP_FILTER_ALPHA/BETA with a range of
-6 to +6, inclusive. This is exactly the range specified for the slice
header fields slice_alpha_c0_offset_div2 and slice_beta_offset_div2,
which store half the actual filter offsets FilterOffsetA/B.

Clarify that this control contains the halved offsets.
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/media/uapi/v4l/extended-controls.rst

@@ -1117,10 +1117,16 @@ enum v4l2_mpeg_video_h264_loop_filter_mode -
 
 ``V4L2_CID_MPEG_VIDEO_H264_LOOP_FILTER_ALPHA (integer)``
     Loop filter alpha coefficient, defined in the H264 standard.
+    This value corresponds to the slice_alpha_c0_offset_div2 slice header
+    field, and should be in the range of -6 to +6, inclusive. The actual alpha
+    offset FilterOffsetA is twice this value.
     Applicable to the H264 encoder.
 
 ``V4L2_CID_MPEG_VIDEO_H264_LOOP_FILTER_BETA (integer)``
     Loop filter beta coefficient, defined in the H264 standard.
+    This corresponds to the slice_beta_offset_div2 slice header field, and
+    should be in the range of -6 to +6, inclusive. The actual beta offset
+    FilterOffsetB is twice this value.
     Applicable to the H264 encoder.
 
 .. _v4l2-mpeg-video-h264-entropy-mode: