release notes: mention iproute2 module

nixos/doc/manual/release-notes/rl-1809.xml

@@ -53,7 +53,10 @@ $ nix-instantiate -E '(import <nixpkgsunstable> {}).gitFull'
 
   <itemizedlist>
    <listitem>
-    <para></para>
+     <para>When enabled the <literal>iproute2</literal> will copy the files
+       expected by ip route (e.g., <filename>rt_tables</filename>) in
+       <filename>/run/iproute2</filename>. This allows to write aliases for
+       routing tables for instance.</para>
    </listitem>
   </itemizedlist>
  </section>