Fix `no_tls_hosts` related docs & tests

[Young-Lord]

Changes proposed in this pull request:

• Add tests for no_tls_hosts config entry
• Improve docs for no_tls_hosts so that it's less surprising

Checklist:

• [x] docs updated
• [x] tests updated
• [x] Changes updated

I am new to Haraka and may be slow to respond, so please feel free to make any necessary modifications. Thanks for your awesome work!