arcbox: Document that implementing weak references via the clear_func is not safe

glib/garcbox.c

@@ -207,6 +207,11 @@ g_atomic_rc_box_release (gpointer mem_block)
  * to clear the contents of @mem_block, and then will free the
  * resources allocated for @mem_block.
  *
+ * Note that implementing weak references via @clear_func is not thread-safe:
+ * clearing a pointer to the memory from the callback can race with another
+ * thread trying to access it as @mem_block already has a reference count of 0
+ * when the callback is called and will be freed.
+ *
  * Since: 2.58
  */
 void