Make sure "pre auth" article mentions that you need an anonymous firewall

[weaverryan]

Hi guys!

See symfony/symfony#13751 - if you're going to return null from createToken(), you must have anonymous on in your firewall so that you at least get the AnonymousToken.

Thanks!

[xabbuh]

reopening here as we may want to put some more clarification here (see #5943 (comment))