[gtk+] Update the migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit b86816438147ffe61959218524f59c44581ee006
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Nov 7 14:59:23 2016 -0500

    Update the migration guide
    
    Mention some removed CSS extensions.

 docs/reference/gtk/migrating-3to4.xml |   18 +++++++++++++++++-
 1 files changed, 17 insertions(+), 1 deletions(-)
---
diff --git a/docs/reference/gtk/migrating-3to4.xml b/docs/reference/gtk/migrating-3to4.xml
index e8ce306..c9d946e 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 +141,22 @@
       </para>
     </section>
 
+    <section>
+      <title>Don't use symbolic colors in your CSS</title>
+      <para>
+        GTK+ 4 does not support @define-color and symbolic colors anymore.
+        We recommend that you use a css preprocessor like sass instead.
+      </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>