[gtk+] Mention pixbufs in the migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit 4f6cee3ee38b2f9c766d2aca942dec401238d41d
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Oct 23 18:33:15 2017 +0200

    Mention pixbufs in the migration guide
    
    They are being phased out.

 docs/reference/gtk/migrating-3to4.xml |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)
---
diff --git a/docs/reference/gtk/migrating-3to4.xml b/docs/reference/gtk/migrating-3to4.xml
index 9b1fb89..e86139a 100644
--- a/docs/reference/gtk/migrating-3to4.xml
+++ b/docs/reference/gtk/migrating-3to4.xml
@@ -295,6 +295,14 @@
         that you are affected by that.
       </para>
     </section>
+
+    <section>
+      <title>GdkPixbuf is deemphasized</title>
+      <para>
+        A number of GdkPixbuf-based APIs have been removed. Use the available
+        surface-based replacements instead.
+      </para>
+    </section>
   </section>
 
 </chapter>