[HttpFoundation] added some information in a phpdoc

src/Symfony/Component/HttpFoundation/SessionStorage/ArraySessionStorage.php

@@ -12,7 +12,9 @@
 namespace Symfony\Component\HttpFoundation\SessionStorage;
 
 /**
- * ArraySessionStorage.
+ * ArraySessionStorage mocks the session for unit tests.
+ *
+ * When doing functional testing, you should use FilesystemArrayStorage instead.
  *
  * @author Fabien Potencier <fabien@symfony.com>
  * @author Bulat Shakirzyanov <mallluhuct@gmail.com>