Add dynamic range comment to vp9_int_pro_col

Change-Id: If14d9f874bd0bf2c5a455982088fd70591f5ea5a

vp9/encoder/vp9_avg.c

@@ -142,9 +142,11 @@ void vp9_int_pro_row_c(int16_t *hbuf, uint8_t const *ref,
   }
 }
 
+// width: value range {16, 32, 64}.
 int16_t vp9_int_pro_col_c(uint8_t const *ref, const int width) {
   int idx;
   int16_t sum = 0;
+  // sum: 14 bit, dynamic range [0, 16320]
   for (idx = 0; idx < width; ++idx)
     sum += ref[idx];
   return sum;