[gtk] migration guide: Add a paragraph about GtkBox's pack-type

[Matthias Clasen <matthiasc src gnome org>]

commit c7b1df1ebdd53d4b111c8532f51ec04dbc23cc60
Author: Timm Bäder <mail baedert org>
Date:   Sat Dec 29 09:53:16 2018 +0100

    migration guide: Add a paragraph about GtkBox's pack-type

 docs/reference/gtk/migrating-3to4.xml | 11 +++++++++++
 1 file changed, 11 insertions(+)
---
diff --git a/docs/reference/gtk/migrating-3to4.xml b/docs/reference/gtk/migrating-3to4.xml
index fcb430adad..111f6f6975 100644
--- a/docs/reference/gtk/migrating-3to4.xml
+++ b/docs/reference/gtk/migrating-3to4.xml
@@ -193,6 +193,17 @@
       </para>
     </section>
 
+    <section>
+      <title>Stop using GtkBox's pack-type child property</title>
+      <para>
+        In order to improve performance and simplify the widget, GtkBox lost its
+        'pack-type' child property. In GTK+4, every GtkBox instance has a simple
+        list of child widgets that it allocates from start to end.
+        The old behavior of pack-type=END can be emulated by simply making the
+        box child in the center hexpand and right-align the ones at the end.
+      </para>
+    </section>
+
   </section>
 
   <section>