[gtk+] Update the migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit 5b8646c6e83cb92fca370ec576859c93b65eff8d
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Nov 7 14:59:23 2016 -0500

    Update the migration guide
    
    Mention the demise of -gtk-gradient, and explain that CSS syntax
    works fine for this now.

 docs/reference/gtk/migrating-3to4.xml |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)
---
diff --git a/docs/reference/gtk/migrating-3to4.xml b/docs/reference/gtk/migrating-3to4.xml
index e8ce306..2638c7d 100644
--- a/docs/reference/gtk/migrating-3to4.xml
+++ b/docs/reference/gtk/migrating-3to4.xml
@@ -133,7 +133,7 @@
     <section>
       <title>Adapt to GtkWidget's size request changes</title>
       <para>
-        GTK+3 used five different virtual functions in GtkWidget to
+        GTK+ 3 used five different virtual functions in GtkWidget to
         implement size requisition, namely the gtk_widget_get_preferred_width()
         family of functions. To simplify widget implementations, GTK+4 uses
         only one virtual function, GtkWidgetClass::measure() that widgets
@@ -141,6 +141,14 @@
       </para>
     </section>
 
+    <section>
+      <title>Don't use -gtk-gradient in your CSS</title>
+      <para>
+        GTK+ now supports standard CSS syntax for both linear and radial
+        gradients, just use those.
+      </para>
+    </section>
+
   </section>
 
 </chapter>