[gtk] Migration guide: mention size-allocate

[Matthias Clasen <matthiasc src gnome org>]

commit 73b45ec77e3dc00d70e59ceee0b264beb6e57bec
Author: Matthias Clasen <mclasen redhat com>
Date:   Sun Jul 29 09:10:44 2018 -0400

    Migration guide: mention size-allocate
    
    Document the new argument here.
    
    Closes: https://gitlab.gnome.org/GNOME/gtk/issues/1245

 docs/reference/gtk/migrating-3to4.xml | 9 +++++++++
 1 file changed, 9 insertions(+)
---
diff --git a/docs/reference/gtk/migrating-3to4.xml b/docs/reference/gtk/migrating-3to4.xml
index 40c0dde97f..f1d29a52b9 100644
--- a/docs/reference/gtk/migrating-3to4.xml
+++ b/docs/reference/gtk/migrating-3to4.xml
@@ -319,6 +319,15 @@
       </para>
     </section>
 
+    <section>
+      <title>Adapt to GtkWidget's size allocation changes</title>
+      <para>
+        The #GtkWidget::size-allocate signal now takes the baseline as an
+        argument, so you no longer need to call gtk_widget_get_allocated_baseline()
+        to get it.
+      </para>
+    </section>
+
     <section>
       <title>Switch to GtkWidget's children APIs</title>
       <para>