docs: add Gtk*Class docs

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

gtk/gtkcellrenderer.h

@@ -90,10 +90,29 @@ struct _GtkCellRenderer
   GtkCellRendererPrivate *priv;
 };
 
+/**
+ * GtkCellRendererClass:
+ * @get_request_mode: Called to gets whether the cell renderer prefers
+ *    a height-for-width layout or a width-for-height layout.
+ * @get_preferred_width: Called to get a renderer's natural width.
+ * @get_preferred_height_for_width: Called to get a renderer's natural height for width.
+ * @get_preferred_height: Called to get a renderer's natural height.
+ * @get_preferred_width_for_height: Called to get a renderer's natural width for height.
+ * @get_aligned_area: Called to get the aligned area used by @cell inside @cell_area.
+ * @get_size: Called to get the width and height needed to render the cell. Deprecated: 3.0.
+ * @render: Called to render the content of the #GtkCellRenderer.
+ * @activate: Called to activate the content of the #GtkCellRenderer.
+ * @start_editing: Called to initiate editing the content of the #GtkCellRenderer.
+ * @editing_canceled: Signal gets emitted when the user cancels the process of editing a cell.
+ * @editing_started: Signal gets emitted when a cell starts to be edited.
+ */
 struct _GtkCellRendererClass
 {
+  /*< private >*/
   GInitiallyUnownedClass parent_class;
 
+  /*< public >*/
+
   /* vtable - not signals */
   GtkSizeRequestMode (* get_request_mode)                (GtkCellRenderer      *cell);
   void               (* get_preferred_width)             (GtkCellRenderer      *cell,
@@ -153,6 +172,8 @@ struct _GtkCellRendererClass
 			     GtkCellEditable *editable,
 			     const gchar     *path);
 
+  /*< private >*/
+
   GtkCellRendererClassPrivate *priv;
 
   /* Padding for future expansion */