[gtk+] Add a note about GDK_AXIS_X/Y

[Matthias Clasen <matthiasc src gnome org>]

commit 7dc588c4d377dbfe54061f35dc67f35cefac9c03
Author: Matthias Clasen <mclasen redhat com>
Date:   Sat Apr 9 17:35:58 2016 -0400

    Add a note about GDK_AXIS_X/Y
    
    These axes mmay or may not be present, best to ignore them.

 gdk/gdktypes.h |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/gdk/gdktypes.h b/gdk/gdktypes.h
index 690cc49..7c4a2bb 100644
--- a/gdk/gdktypes.h
+++ b/gdk/gdktypes.h
@@ -546,6 +546,10 @@ typedef enum
  * An enumeration describing the way in which a device
  * axis (valuator) maps onto the predefined valuator
  * types that GTK+ understands.
+ *
+ * Note that the X and Y axes are not really needed; pointer devices
+ * report their location via the x/y members of events regardless. Whether
+ * X and Y are present as axes depends on the GDK backend.
  */
 typedef enum
 {