[gtk+] Mention single-stop gradients in the release notes

[Matthias Clasen <matthiasc src gnome org>]

commit 847f9f40cf05edaf1f82d5fea4006076572cc720
Author: Matthias Clasen <mclasen redhat com>
Date:   Thu Jun 9 08:05:11 2016 -0400

    Mention single-stop gradients in the release notes
    
    This is something people stumble over in their custom CSS.

 docs/reference/gtk/migrating-3xtoy.xml |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)
---
diff --git a/docs/reference/gtk/migrating-3xtoy.xml b/docs/reference/gtk/migrating-3xtoy.xml
index 8dd4c57..a3b0a6c 100644
--- a/docs/reference/gtk/migrating-3xtoy.xml
+++ b/docs/reference/gtk/migrating-3xtoy.xml
@@ -405,6 +405,12 @@
     </para>
 
     <para>
+      The CSS parser now warns about some more constructs that are not according
+      to the CSS spec, such as gradients with a single color stop. Instead,
+      you can just use image(&lt;color&gt;).
+    </para>
+
+    <para>
       The #GtkSizeGroup:ignore-hidden property has not been working properly
       for a long time, and we've not documented it as broken and deprecated.
       The recommended alternative for reserving space of widgets that are not