Amend the help text of the TLS certificate

[yubiuser]

What does this PR aim to accomplish?:

Improve the help text of the webserver.tls.cert config.

Fixes https://discourse.pi-hole.net/t/v6-self-signed-certificate-issues/73640

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[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/v6-self-signed-certificate-issues/73640/9

[DL6ER]

You'll also need to edit the test/pihole.toml file as the tests expect the file to be created correctly

[yubiuser]

You'll also need to edit the test/pihole.toml file as the tests expect the file to be created correctly

I feared so.