diff --git a/gobject/gtype.h b/gobject/gtype.h
index 0d366e67b..70181cd5c 100644
--- a/gobject/gtype.h
+++ b/gobject/gtype.h
@@ -1753,6 +1753,12 @@ guint     g_type_get_type_registration_serial (void);
  * name `t_n ## _default_init`, and the interface structure to have the
  * name `TN ## Interface`.
  *
+ * The initialization function has signature
+ * `static void t_n ## _default_init (TypeName##Interface *klass);`, rather than
+ * the full #GInterfaceInitFunc signature, for brevity and convenience. If you
+ * need to use an initialization function with an `iface_data` argument, you
+ * must write the #GTypeInterface definitions manually.
+ *
  * Since: 2.24
  */
 #define G_DEFINE_INTERFACE(TN, t_n, T_P)		    G_DEFINE_INTERFACE_WITH_CODE(TN, t_n, T_P, ;)