_Py_normalize_encoding(): explain how the value 6 was computed

Objects/unicodeobject.c

@@ -2983,6 +2983,7 @@ _Py_normalize_encoding(const char *encoding,
     char *l_end;
 
     if (encoding == NULL) {
+        /* 6 == strlen("utf-8") + 1 */
         if (lower_len < 6)
             return 0;
         strcpy(lower, "utf-8");