This is no longer a supported element in ui files.
* content below <child>. To handle an element, the implementation
* must fill in the @parser and @user_data and return %TRUE.
* #GtkWidget implements this to parse keyboard accelerators specified
- * in <accelerator> elements. #GtkContainer implements it to map
- * properties defined via <packing> elements to child properties.
+ * in <accelerator> elements.
* Note that @user_data must be freed in @custom_tag_end or @custom_finished.
* @custom_tag_end: Called for the end tag of each custom element that is
* handled by the buildable (see @custom_tag_start).