docs: add some missing gtk-doc blocks and Since tags for new API

gdk/gdkframeclock.h

@@ -76,6 +76,7 @@ typedef enum {
 
 GType    gdk_frame_clock_get_type             (void) G_GNUC_CONST;
 
+GDK_AVAILABLE_IN_3_8
 gint64   gdk_frame_clock_get_frame_time            (GdkFrameClock *frame_clock);
 
 GDK_AVAILABLE_IN_3_8

gtk/gtkwidget.c

@@ -4632,6 +4632,11 @@ static guint tick_callback_id;
  * This is a more convenient alternative to connecting directly to the
  * ::update signal of GdkFrameClock, since you don't have to worry about
  * when a #GdkFrameClock is assigned to a widget.
+ *
+ * Returns: an id for the connection of this callback. Remove the callback
+ *     by passing it to gtk_widget_remove_tick_callback()
+ *
+ * Since: 3.8
  */
 guint
 gtk_widget_add_tick_callback (GtkWidget       *widget,
@@ -4669,6 +4674,16 @@ gtk_widget_add_tick_callback (GtkWidget       *widget,
   return info->id;
 }
 
+/**
+ * gtk_widget_remove_tick_callback:
+ * @widget: a #GtkWidget
+ * @id: an id returned by gtk_widget_add_tick_callback()
+ *
+ * Removes a tick callback previously registered with
+ * gtk_widget_add_tick_callback().
+ *
+ * Since: 3.8
+ */
 void
 gtk_widget_remove_tick_callback (GtkWidget *widget,
                                  guint      id)

gtk/gtkwidget.h

@@ -96,6 +96,8 @@ typedef void    (*GtkCallback)     (GtkWidget        *widget,
  *
  * Returns: %G_SOURCE_CONTINUE if the tick callback should continue to be called,
  *  %G_SOURCE_REMOVE if the tick callback should be removed.
+ *
+ * Since: 3.8
  */
 typedef gboolean (*GtkTickCallback) (GtkWidget     *widget,
                                      GdkFrameClock *frame_clock,
@@ -488,6 +490,7 @@ void	   gtk_widget_queue_draw_region   (GtkWidget	       *widget,
                                            const cairo_region_t*region);
 void	   gtk_widget_queue_resize	  (GtkWidget	       *widget);
 void	   gtk_widget_queue_resize_no_redraw (GtkWidget *widget);
+GDK_AVAILABLE_IN_3_8
 GdkFrameClock* gtk_widget_get_frame_clock (GtkWidget           *widget);
 
 GDK_DEPRECATED_IN_3_0_FOR(gtk_widget_get_preferred_size)