[gtk+] docs: add missing docs for GDK_WINDOWING_WAYLAND

[William Jon McCann <mccann src gnome org>]

commit dc8439ee79acfbe5a86f6e5bd97548cf1839cfa8
Author: William Jon McCann <william jon mccann gmail com>
Date:   Mon Jan 20 13:42:57 2014 -0500

    docs: add missing docs for GDK_WINDOWING_WAYLAND

 gdk/gdk.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)
---
diff --git a/gdk/gdk.c b/gdk/gdk.c
index 6362d9b..86adb85 100644
--- a/gdk/gdk.c
+++ b/gdk/gdk.c
@@ -89,6 +89,15 @@
  */
 
 /**
+ * GDK_WINDOWING_WAYLAND:
+ *
+ * The #GDK_WINDOWING_WAYLAND macro is defined if the Wayland backend
+ * is supported.
+ *
+ * Use this macro to guard code that is specific to the Wayland backend.
+ */
+
+/**
  * GDK_DISABLE_DEPRECATION_WARNINGS:
  *
  * A macro that should be defined before including the gdk.h header.