Update session.rst

May be a bug, but Sessions expire after 30 minutes (or less ?)
To have the default behavior describe in documentation, you must set cookie_lifetime to 0

$app->register(new Silex\Provider\UrlGeneratorServiceProvider(), array(
    'session.storage.options' => array(
        'cookie_lifetime' => 0
        )
    )
);

doc/providers/session.rst

@@ -26,8 +26,7 @@ Parameters
   * **cookie_secure**: Cookie secure (HTTPS)
   * **cookie_httponly**: Whether the cookie is http only
 
-  However, all of these are optional. Sessions last as long as the browser is
-  open. To override this, set the ``lifetime`` option.
+  However, all of these are optional. Default Sessions life time is 1800 seconds (30 minutes). To override this, set the ``lifetime`` option.
 
   For a full list of available options, read the `PHP
   <http://php.net/session.configuration>`_ official documentation.