[gtk+] docs: Add a note about button 4 & 5

[Matthias Clasen <matthiasc src gnome org>]

commit 71bd0b05ad7eb4479f7632d3dffdfc92699ec184
Author: Matthias Clasen <mclasen redhat com>
Date:   Sat May 2 11:55:00 2015 -0400

    docs: Add a note about button 4 & 5

 gdk/gdktypes.h |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/gdk/gdktypes.h b/gdk/gdktypes.h
index 7f45cee..64e7243 100644
--- a/gdk/gdktypes.h
+++ b/gdk/gdktypes.h
@@ -207,6 +207,10 @@ typedef enum
  * reserved values such as %GDK_MODIFIER_RESERVED_13_MASK.  Your code
  * should preserve and ignore them.  You can use %GDK_MODIFIER_MASK to
  * remove all reserved values.
+ *
+ * Also note that the GDK X backend interprets button press events for button
+ * 4-7 as scroll events, so %GDK_BUTTON4_MASK and %GDK_BUTTON5_MASK will never
+ * be set.
  */
 typedef enum
 {