docs: add Gtk*Class docs

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

gtk/deprecated/gtkactiongroup.h

@@ -60,6 +60,11 @@ struct _GtkActionGroup
   GtkActionGroupPrivate *priv;
 };
 
+/**
+ * GtkActionGroupClass:
+ * @parent_class: The parent class.
+ * @get_action: Looks up an action in the action group by name.
+ */
 struct _GtkActionGroupClass
 {
   GObjectClass parent_class;
@@ -67,6 +72,8 @@ struct _GtkActionGroupClass
   GtkAction *(* get_action) (GtkActionGroup *action_group,
                              const gchar    *action_name);
 
+  /*< private >*/
+
   /* Padding for future expansion */
   void (*_gtk_reserved1) (void);
   void (*_gtk_reserved2) (void);