Add a comment for valid_ref_frame_size()

Point to the corresponding section in the AV1 spec.

The valid_ref_frame_size() function comes from libvpx.

The valid_ref_frame_size() function was originally added as the
check_scale_factors() function in
https://chromium-review.googlesource.com/c/webm/libvpx/+/66531

It was moved to the current location and renamed in
https://chromium-review.googlesource.com/c/webm/libvpx/+/70632

Change-Id: I43320afecdd2238e72512190fece56116b6ecd17

av1/common/scale.h

diff --git a/av1/common/scale.h b/av1/common/scale.h
index fd30416..28bcec4 100644
--- a/av1/common/scale.h
+++ b/av1/common/scale.h
@@ -52,6 +52,7 @@
          (sf->x_scale_fp != REF_NO_SCALE || sf->y_scale_fp != REF_NO_SCALE);
 }
 
+// See AV1 spec, Section 6.8.6. Frame size with refs semantics.
 static INLINE int valid_ref_frame_size(int ref_width, int ref_height,
                                        int this_width, int this_height) {
   return 2 * this_width >= ref_width && 2 * this_height >= ref_height &&