Add tip about blocking garbage requests

[Anuskuss]

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What does this PR aim to accomplish?: Let the user know about a free optimization in a setup where the Pi-hole is not the DHCP server.

How does this PR accomplish the above?: This RegExp prevents the forwarding of bad requests which are not intended to be solved by the DHCP server.

What documentation changes (if any) are needed to support this PR?: https://discourse.pi-hole.net/t/dont-forward-bad-hostnames/49634

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