[gtk+] docs: Add a release note about GtkSizeGroup::ignore-hidden

[Matthias Clasen <matthiasc src gnome org>]

commit dbe499bcfef2504204c24eda6404bf2269fda67a
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue May 31 16:33:18 2016 -0400

    docs: Add a release note about GtkSizeGroup::ignore-hidden

 docs/reference/gtk/migrating-3xtoy.xml |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)
---
diff --git a/docs/reference/gtk/migrating-3xtoy.xml b/docs/reference/gtk/migrating-3xtoy.xml
index 19fd608..8dd4c57 100644
--- a/docs/reference/gtk/migrating-3xtoy.xml
+++ b/docs/reference/gtk/migrating-3xtoy.xml
@@ -403,5 +403,13 @@
       change an individual facet of the font, like the weight, use the
       individual CSS properties: font-weight, font-size, font-family, etc.
     </para>
+
+    <para>
+      The #GtkSizeGroup:ignore-hidden property has not been working properly
+      for a long time, and we've not documented it as broken and deprecated.
+      The recommended alternative for reserving space of widgets that are not
+      currently shown in the UI is to use a #GtkStack (with some 'filler'
+      widget, e.g. an empty #GtkBox, in another page).
+    </para>
   </section>
 </chapter>