networking.config API: Add an example to the documentation.

extensions/common/api/networking_config.idl

Index: extensions/common/api/networking_config.idl
diff --git a/extensions/common/api/networking_config.idl b/extensions/common/api/networking_config.idl
index 964fc851165d6b542891766591cc6bc59ceb4f3b..78b5bc6653efb27770ec5bd806a591e4cc18ff63 100644
--- a/extensions/common/api/networking_config.idl
+++ b/extensions/common/api/networking_config.idl
@@ -84,7 +84,7 @@ namespace networking.config {
     // |result|: The result of the authentication attempt.
     // |callback|: Called back when this operation is finished.
     void finishAuthentication(DOMString GUID, AuthenticationResult result,
-                              FinishAuthenticationCallback callback);
+                              optional FinishAuthenticationCallback callback);
   };
 
   interface Events {