Revert "Removed warning about using swiftmailer handler with memory spool"

This reverts commit 1262487380d02ac3239b452ff4a5ef4f1d683342.

There is [apparently no way](https://github.com/symfony/symfony-docs/commit/1262487380d02ac3239b452ff4a5ef4f1d683342#commitcomment-4393759) people in 2.3 can get this bugfix. Even when installing MonologBundle 2.4.1 , it does not work for me.