[gtk+] Docs: Mention that shadow type is a lie

[Matthias Clasen <matthiasc src gnome org>]

commit 5f549fa7cb3c1cd4ea288386c4d6fbe41f42c614
Author: Matthias Clasen <mclasen redhat com>
Date:   Thu Jul 10 00:16:04 2014 -0400

    Docs: Mention that shadow type is a lie
    
    As requested here:
    https://bugzilla.gnome.org/show_bug.cgi?id=659926#c2

 gtk/gtkenums.h |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/gtk/gtkenums.h b/gtk/gtkenums.h
index 3c9bd12..aa893d6 100644
--- a/gtk/gtkenums.h
+++ b/gtk/gtkenums.h
@@ -467,6 +467,10 @@ typedef enum
  * @GTK_SHADOW_ETCHED_OUT: The outline has a raised 3d appearance.
  *
  * Used to change the appearance of an outline typically provided by a #GtkFrame.
+ *
+ * Note that many themes do not differentiate the appearance of the
+ * various shadow types: Either their is no visible shadow (@GTK_SHADOW_NONE),
+ * or there is (any other value).
  */
 typedef enum
 {