Add a note for enum values of MV_REFERENCE_FRAME

Change-Id: Ifaf6738f26e86ded6eb6ea1465bad7a229612999

vp9/common/vp9_blockd.h

@@ -98,6 +98,9 @@ typedef struct {
   int_mv as_mv[2];  // first, second inter predictor motion vectors
 } b_mode_info;
 
+// Note that the rate-distortion optimization loop, bit-stream writer, and
+// decoder implementation modules critically rely on the enum entry values
+// specified herein. They should be refactored concurrently.
 typedef enum {
   NONE = -1,
   INTRA_FRAME = 0,