Warn of environmental variable inclusion in phpinfo/super globals dump

[michaelcullum]

We should warn in the docs that dumping $_SERVER, $_ENV, or outputting phpinfo() would disclose contents of environmental variables which, as of Symfony 4 best practices, would include database credentials.

Originally commented upon: https://twitter.com/mdekrijger/status/958688583858212865

[javiereguiluz]

Fixed by #9245.