[gtk/no-app-menu] docs: Mention app menus in the migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit 1038b9b8a1b88473d526bfad2c0890ff8af06923
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri May 22 18:46:51 2020 -0400

    docs: Mention app menus in the migration guide

 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 e65d19befe..0b949d24f6 100644
--- a/docs/reference/gtk/migrating-3to4.xml
+++ b/docs/reference/gtk/migrating-3to4.xml
@@ -245,6 +245,17 @@
         the amount of work you have to do at the time of the switch.
       </para>
     </section>
+
+    <section>
+      <title>Stop using app menus</title>
+      <para>
+        GNOME has stopped using exported app menus some time ago, and GTK 4 is
+        dropping the GtkApplication APIs that are related to app menus. If you
+        want to transparently support menubars on OS X, you can integrate the
+        items from your app menu into your menubar, and keep using
+        gtk_application_set_menubar().
+      </para>
+    </section>
   </section>
 
   <section>