[gtk/wip/ebassi/fixed-layout-v2] docs: Mention GtkLayout's removal in the migration guide

[Emmanuele Bassi <ebassi src gnome org>]

commit 167f49caf7c1828501118c0d6db750f19e458e9a
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Tue Apr 2 16:00:05 2019 +0100

    docs: Mention GtkLayout's removal in the migration guide

 docs/reference/gtk/migrating-3to4.xml | 10 ++++++++++
 1 file changed, 10 insertions(+)
---
diff --git a/docs/reference/gtk/migrating-3to4.xml b/docs/reference/gtk/migrating-3to4.xml
index 60afd4d7c2..8a6d09c5d3 100644
--- a/docs/reference/gtk/migrating-3to4.xml
+++ b/docs/reference/gtk/migrating-3to4.xml
@@ -665,6 +665,16 @@
         pass-through == !can-pick.
       </para>
     </section>
+
+    <section>
+      <title>Use GtkFixed instead of GtkLayout</title>
+      <para>
+        Since GtkScrolledWindow can deal with widgets that do not implement
+        the GtkScrollable interface by automatically wrapping them into a
+        GtkViewport, GtkLayout is redundant, and has been removed in favor
+        of the existing GtkFixed containter widget.
+      </para>
+    </section>
   </section>
 
 </chapter>