[gtk+] Add a release note for scrolled windows

[Matthias Clasen <matthiasc src gnome org>]

commit 76af90767634f5caea622c2abc13d21adb41550c
Author: Matthias Clasen <mclasen redhat com>
Date:   Sun Jun 19 22:23:06 2016 -0400

    Add a release note for scrolled windows
    
    Mention the changed sizing behavior and how to address it.

 docs/reference/gtk/migrating-3xtoy.xml |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)
---
diff --git a/docs/reference/gtk/migrating-3xtoy.xml b/docs/reference/gtk/migrating-3xtoy.xml
index a3b0a6c..b546cba 100644
--- a/docs/reference/gtk/migrating-3xtoy.xml
+++ b/docs/reference/gtk/migrating-3xtoy.xml
@@ -417,5 +417,14 @@
       currently shown in the UI is to use a #GtkStack (with some 'filler'
       widget, e.g. an empty #GtkBox, in another page).
     </para>
+
+    <para>
+      GtkScrolledWindow now respects the natural size of its child and passes
+      it along. This may change the size distribution of layouts involving
+      scrolled windows (they may take up more space than before). In most
+      cases, setting suitable values for #GtkScrolledWindow:max-content-width
+      and #GtkScrolledWindow:max-content-height will allow control over the
+      size of the scrolled window.
+    </para>
   </section>
 </chapter>