[gtk+/gtk-3-20] Update migration guide a bit

[Matthias Clasen <matthiasc src gnome org>]

commit f7e57b5eebb3a193bf73e383a6cab96912fd1661
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue May 24 20:55:16 2016 -0400

    Update migration guide a bit
    
    Resize grips are no more.

 docs/reference/gtk/migrating-2to3.xml |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)
---
diff --git a/docs/reference/gtk/migrating-2to3.xml b/docs/reference/gtk/migrating-2to3.xml
index b0bad87..26a96e8 100644
--- a/docs/reference/gtk/migrating-2to3.xml
+++ b/docs/reference/gtk/migrating-2to3.xml
@@ -1205,6 +1205,12 @@ gtk_arrow_draw (GtkWidget *widget,
      have disappeared, and instead there are now
      gtk_window_set_has_resize_grip() and gtk_window_get_has_resize_grip().
     </para>
+    <para>
+     In more recent versions of GTK+ 3, the resize grip functionality has
+     been removed entirely, in favor of invisible resize borders around the
+     window. When updating to newer versions of GTK+ 3, you should simply
+     remove all code dealing with resize grips.
+    </para>
   </section>
 
   <section>