Docs: Mention that shadow type is a lie

As requested here:
https://bugzilla.gnome.org/show_bug.cgi?id=659926#c2

diff --git a/gtk/gtkenums.h b/gtk/gtkenums.h
index 3c9bd121fad4cbf9a9cf8c9bd472b93f9ddf1f67..aa893d6b402e1f9ac9bbe283c2d547a1415ce93d 100644 (file)
--- 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
 {