Mention GTK_DIALOG_NO_SEPARATOR in migration docs

http://bugzilla.gnome.org/show_bug.cgi?id=647779

docs/reference/gtk/migrating-2to3.xml

@@ -94,6 +94,13 @@
     make CFLAGS+="-DGDK_DISABLE_DEPRECATED -DGTK_DISABLE_DEPRECATED"
     </programlisting>
   </para>
+  <para>
+    Note that some parts of our API, such as enumeration values, are
+    not well covered by the deprecation warnings. In most cases, using
+    them will require you to also use deprecated functions, which will
+    trigger warnings. But some things, like the %GTK_DIALOG_NO_SEPARATOR
+    flag that has disappeared in GTK+ 3, may not. 
+  </para>
   </section>
 
   <section>