[gtk+] Add border-width to the migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit 9a684accfea1b786d8a3b36d97f339ce0ea2e236
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Oct 21 09:58:36 2016 -0400

    Add border-width to the migration guide
    
    No good answers here yet.

 docs/reference/gtk/migrating-3to4.xml |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)
---
diff --git a/docs/reference/gtk/migrating-3to4.xml b/docs/reference/gtk/migrating-3to4.xml
index b818a7f..d45b688 100644
--- a/docs/reference/gtk/migrating-3to4.xml
+++ b/docs/reference/gtk/migrating-3to4.xml
@@ -121,6 +121,15 @@
       </para>
     </section>
 
+    <section>
+      <title>Stop using GtkContainer::border-width</title>
+      <para>
+        GTK+ 4 has removed the #GtkContainer::border-width property.
+        Use other means to influence the spacing of your containers.
+        FIXME: what are those ?
+      </para>
+    </section>
+
   </section>
 
 </chapter>