[gtk+] Document Wayland opacity handling

[Matthias Clasen <matthiasc src gnome org>]

commit b673490a1b492308462b97569941c3834a06214f
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Oct 28 10:48:49 2014 -0400

    Document Wayland opacity handling

 gdk/gdkwindow.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)
---
diff --git a/gdk/gdkwindow.c b/gdk/gdkwindow.c
index 60401a3..a4e2bbb 100644
--- a/gdk/gdkwindow.c
+++ b/gdk/gdkwindow.c
@@ -10531,7 +10531,12 @@ gdk_window_configure_finished (GdkWindow *window)
  *
  * For toplevel windows this depends on support from the windowing system
  * that may not always be there. For instance, On X11, this works only on
- * X screens with a compositing manager running.
+ * X screens with a compositing manager running. On Wayland, there is no
+ * per-window opacity value that the compositor would apply. Instead, use
+ * `gdk_window_set_opaque_region (window, NULL)` to tell the compositor
+ * that the entire window is (potentially) non-opaque, and draw your content
+ * with alpha, or use gtk_widget_set_opacity() to set an overall opacity
+ * for your widgets.
  *
  * For child windows this function only works for non-native windows.
  *