Document check for 30176, since it's a bit subtle.

src/test/test_containers.c

@@ -1006,6 +1006,9 @@ test_container_smartlist_remove(void *arg)
   tt_ptr_op(smartlist_get(sl, 1), OP_EQ, &array[2]);
   tt_ptr_op(smartlist_get(sl, 2), OP_EQ, &array[1]);
   tt_ptr_op(smartlist_get(sl, 3), OP_EQ, &array[2]);
+  /* Ordinary code should never look at this pointer; we're doing it here
+   * to make sure that we really cleared the pointer we removed.
+   */
   tt_ptr_op(sl->list[4], OP_EQ, NULL);
 
  done: