Add link to Networking extensions

The sentence was incomplete, finish it and add proper link.
Remove also a broken link.

Change-Id: Iae7616b5ebd327f1ea68302e5e29f4f85f8ec904
Closes-Bug: #1368485

v2.0/ch_neutron_general_info.xml

@@ -49,9 +49,8 @@
         for.</para>
     <section xml:id="Authentication-d1e444">
         <title>Authentication and authorization</title>
-        <para>The &APIv2; uses the <link
-                xlink:href="https://openstack.keystone.org">Keystone
-                Identity Service</link> as the default authentication
+        <para>The &APIv2; uses the Keystone
+                Identity Service as the default authentication
             service. When Keystone is enabled, users that submit
             requests to the OpenStack Networking service must provide
             an authentication token in <emphasis role="bold"
@@ -395,8 +394,9 @@ Accept: application/xml
             be supported by all plug-ins, the availability of an
             extension varies with deployments and the specific plug-in
             in use.</para>
-        <para>For more information regarding specific extensions, see
-            <!-- <xref linkend="API_extensions"/>--></para>
+        <para>For information about specific extensions, see
+            <link xlink:href="http://developer.openstack.org/api-ref-networking-v2.html#netconn-v2-ext">Networking API v2.0 extensions</link>.
+        </para>
     </section>
     <?hard-pagebreak?>
     <section xml:id="faults">