Bug 1397440 - Added a very useful comment r=rillian

This is a practice commit to clarify what arguments we're accepting.

MozReview-Commit-ID: 2qQbNAYzGwr

--HG--
extra : rebase_source : 26527c08c32d79f794601afdd7832fa0ed53ecf7

dom/media/platforms/agnostic/AOMDecoder.cpp

@@ -138,6 +138,7 @@ highbd_img_downshift(aom_image_t *dst, aom_image_t *src, int down_shift) {
     case AOM_IMG_FMT_I44016:
       break;
     default:
+      // We don't support anything that's not 16 bit
       return AOM_CODEC_UNSUP_BITSTREAM;
   }
   for (plane = 0; plane < 3; plane++) {