[gtk] Mention gtk_widget_show_all in the migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit 0ba307995d231d58c147c3f82e47be5ee0ab5455
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Jul 31 13:29:32 2018 -0400

    Mention gtk_widget_show_all in the migration guide
    
    This function is gone.
    
    Closes https://gitlab.gnome.org/GNOME/gtk/issues/1242

 docs/reference/gtk/migrating-3to4.xml | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/docs/reference/gtk/migrating-3to4.xml b/docs/reference/gtk/migrating-3to4.xml
index f1d29a52b9..cd113bb79b 100644
--- a/docs/reference/gtk/migrating-3to4.xml
+++ b/docs/reference/gtk/migrating-3to4.xml
@@ -408,6 +408,10 @@
         from ui files it run the command <command>gtk4-builder-tool simplify --replace</command>
         on them.
       </para>
+      <para>
+        The function gtk_widget_show_all(), the #GtkWidget::no-show-all property
+        and its getter and setter have been removed in  GTK+ 4, so you should stop using them.
+      </para>
     </section>
 
     <section>