Add (out) annotation and documentation to meta_window_get_outer_rectangle()

GObject introspection cannot detect that rect is an (out) parameter.
Also add a meaningful documentation for humans like us.

src/core/window.c

@@ -4691,6 +4691,13 @@ meta_window_get_geometry (MetaWindow  *window,
     window->size_hints.height_inc;
 }
 
+/**
+ * meta_window_get_outer_rect:
+ * @window: a #MetaWindow
+ * @rect: (out): pointer to an allocated #MetaRectangle
+ *
+ * Gets the rectangle that bounds @window and, if decorated, its decorations.
+ */
 void
 meta_window_get_outer_rect (const MetaWindow *window,
                             MetaRectangle    *rect)