docs: add Gtk*Class docs

https://bugzilla.gnome.org/show_bug.cgi?id=81006

gtk/gtkbbox.h

@@ -54,10 +54,16 @@ struct _GtkButtonBox
   GtkButtonBoxPrivate *priv;
 };
 
+/**
+ * GtkButtonBoxClass:
+ * @parent_class: The parent class.
+ */
 struct _GtkButtonBoxClass
 {
   GtkBoxClass parent_class;
 
+  /*< private >*/
+
   /* Padding for future expansion */
   void (*_gtk_reserved1) (void);
   void (*_gtk_reserved2) (void);