Improve the docs for gdk_window_set_opaque_region

Fix several typos and annotate the region parameter
as allow-none.

gdk/gdkwindow.c

@@ -11100,9 +11100,9 @@ gdk_window_get_unscaled_size (GdkWindow *window,
 /**
  * gdk_window_set_opaque_region:
  * @window: a top-level or non-native #GdkWindow
- * @region: a region
+ * @region: (allow-none):  a region, or %NULL
  *
- * For optimizization purposes, compositing window managers may
+ * For optimisation purposes, compositing window managers may
  * like to not draw obscured regions of windows, or turn off blending
  * during for these regions. With RGB windows with no transparency,
  * this is just the shape of the window, but with ARGB32 windows, the
@@ -11111,7 +11111,7 @@ gdk_window_get_unscaled_size (GdkWindow *window,
  *
  * This function only works for toplevel windows.
  *
- * GTK+ will automatically update this property automatically if
+ * GTK+ will update this property automatically if
  * the @window background is opaque, as we know where the opaque regions
  * are. If your window background is not opaque, please update this
  * property in your #GtkWidget::style-updated handler.