Add annotation to gdk_screen_get_monitor_geometry

Pointed out by William Jon McCann.

gdk/x11/gdkscreen-x11.c

@@ -450,7 +450,7 @@ gdk_x11_screen_get_monitor_output (GdkScreen *screen,
  * gdk_screen_get_monitor_geometry:
  * @screen: a #GdkScreen
  * @monitor_num: the monitor number, between 0 and gdk_screen_get_n_monitors (screen)
- * @dest: a #GdkRectangle to be filled with the monitor geometry
+ * @dest: (out) (allow-none): a #GdkRectangle to be filled with the monitor geometry
  *
  * Retrieves the #GdkRectangle representing the size and position of
  * the individual monitor within the entire screen area.