Added the versionadded directive

Author: javiereguiluz

components/http_foundation/session_configuration.rst

@@ -91,6 +91,9 @@ Example usage::
 Migrating Between Save Handlers
 -------------------------------
 
+.. versionadded:: 4.1
+    The ``MigratingSessionHandler`` class was introduced in Symfony 4.1.
+
 If your application changes the way sessions are stored, use the
 :class:`Symfony\\Component\\HttpFoundation\\Session\\Storage\\Handler\\MigratingSessionHandler`
 to migrate between old and new save handlers without losing session data.
@@ -110,7 +113,6 @@ This is the recommended migration workflow:
 
 #. After verifying that the sessions in your application are working, switch
    from the migrating handler to the new handler.
-    $sessionStorage = $newSessionStorage;
 
 Configuring PHP Sessions
 ~~~~~~~~~~~~~~~~~~~~~~~~