Add comment in av1_update_rd_thresh_fact()

Change-Id: I01421950d49039c232a46b0af229103afd29f827
diff --git a/av1/encoder/rd.c b/av1/encoder/rd.c
index 91a912d..55ba611 100644
--- a/av1/encoder/rd.c
+++ b/av1/encoder/rd.c
@@ -1328,6 +1328,7 @@
       min_size = AOMMAX(bsize - 1, BLOCK_4X4);
       max_size = AOMMIN(bsize + 2, (int)cm->seq_params.sb_size);
     } else {
+      // This part handles block sizes with 1:4 and 4:1 aspect ratios
       // TODO(any): Experiment with threshold update for parent/child blocks
       min_size = bsize;
       max_size = bsize;