[gtk+] Add some drawing information the migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit 9ecb34b4a00f4621d4a1b8266544c779b3da9ef5
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Nov 18 07:42:49 2016 -0500

    Add some drawing information the migration guide
    
    Mention snapshot(), and the new GtkDrawingArea API.

 docs/reference/gtk/migrating-3to4.xml |   21 +++++++++++++++++++++
 1 files changed, 21 insertions(+), 0 deletions(-)
---
diff --git a/docs/reference/gtk/migrating-3to4.xml b/docs/reference/gtk/migrating-3to4.xml
index 2638c7d..e8ce2aa 100644
--- a/docs/reference/gtk/migrating-3to4.xml
+++ b/docs/reference/gtk/migrating-3to4.xml
@@ -149,6 +149,27 @@
       </para>
     </section>
 
+    <section>
+      <title>Adapt to drawing model changes</title>
+      <para>
+        This area has seen the most radical changes in the transition from GTK+ 3
+        to GTK+ 4. Widgets no longer use a draw() function to render their contents
+        to a cairo surface. Instead, they have a snapshot() function that creates
+        one or more GskRenderNodes to represent their content. Third-party widgets
+        that use a draw() function or a #GtkWidget::draw signal handler for custom
+        drawing will need to be converted to use gtk_snapshot_append_cairo_node().
+      </para>
+      <para>
+        The auxiliary #GtkSnapshot object has APIs to help with creating render
+        nodes.
+      </para>
+      <para>
+        If you are using a #GtkDrawingArea for custom drawing, you need to switch
+        to using gtk_drawing_area_set_draw_func() to set a draw function. This is
+        pretty much a direct replacement for a #GtkWidget::draw signal handler.
+      </para>
+    </section>
+
   </section>
 
 </chapter>