ARGBToUV_C and ARGBToUVJ_C are generated functions with subtle
difference in rounding.  Adding comment to make them easier to find.

TBR=kjellander@chromium.org
BUG=libyuv:634
TEST=untested

Change-Id: I9912d256a1e04c58475d33bdb472c37484f6cab9
Reviewed-on: https://chromium-review.googlesource.com/434980Reviewed-by: Frank Barchard <fbarchard@google.com>
Commit-Queue: Frank Barchard <fbarchard@google.com>