Updated docs for Monolog "swift" handler in cookbook.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | 2.1+
| Fixed tickets |

.. versionadded:: 2.1
    Support for multiple recipients is a new Monolog feature that's available in
    Symfony 2.1. Prior, only a single email address was accepted as `monolog.handlers.swift.to_email` configuration.