Added a note about the automatic handling of the memory spool in the CLI

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | n/a

since https://github.com/symfony/SwiftmailerBundle/pull/64 flushing the memory spool manually is not necessary anymore when using Symfony 2.3+. I added a versionadded note here, but we might decide to remove the obsolete paragraph instead, making the cookbook only about generating urls (which would require renaming the page then)