Document the migration of Description custom field

Author: sunnavy

docs/UPGRADING-6.0

@@ -65,6 +65,12 @@ C<html/RTIR/Incident/Reply/index.html> to C<html/RTIR/Incident/Elements/ReplyFor
 The callback C<AfterTimeWorked> is moved to a similar location in C<ReplyForm>
 and is called C<AfterWorked>.
 
+=item *
+
+Description custom field has been migrated to RT's core Description
+field. Because of this change, "Description" now has its own widget on
+create/display pages.
+
 =back
 
 =head2 New Lookup Tools