docs: add Gtk*Class docs

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

gtk/gtkswitch.h

@@ -61,17 +61,19 @@ struct _GtkSwitch
 
 /**
  * GtkSwitchClass:
- *
- * The <structname>GtkSwitchClass</structname> structure contains only
- * private data.
+ * @parent_class: The parent class.
+ * @activate: An action signal and emitting it causes the switch to animate.
  */
 struct _GtkSwitchClass
 {
-  /*< private >*/
   GtkWidgetClass parent_class;
 
+  /*< public >*/
+
   void (* activate) (GtkSwitch *sw);
 
+  /*< private >*/
+
   void (* _switch_padding_1) (void);
   void (* _switch_padding_2) (void);
   void (* _switch_padding_3) (void);