added missing service in the doc

doc/services.rst

@@ -236,6 +236,11 @@ of them.
 * **request_context**: The request context is a simplified representation
   of the request that is used by the Router and the UrlGenerator.
 
+* **exception_handler**: The Exception handler is the default handler that is
+  used when you don't register one via the `error()` method or if your handler
+  does not return a Response. Disable it with
+  `unset($app['exception_handler'])`.
+
 .. note::
 
     All of these Silex core services are shared.