Document when macOS changes your hostname

[Similar to #3329, but for macOS]

We should document when macOS changes your host name, as people think Tailscale's the one doing it.

As Dominic Evans writes on Twitter, https://twitter.com/oldmanuk/status/1496066818611548160:

> If you haven’t specified an explicit hostname on your Mac and OSX finds a reverse dns entry for what it believes to be your primary IP address then it will set your hostname to that value automatically.
>
> The only way to prevent that on modern OS X is to ensure you’ve explicitly set your desired hostname everywhere. You can do it in the terminal as so:
>
> export CNAME=xxx
> for n in LocalHostName ComputerName HostName; do sudo scutil --set $n $CNAME; done

For now at least we'll have this bug for people to search for. @WalterHub, up to you if this should be in KB too.

Review issue #3329 and the existing Tailscale documentation or KB guidance about hostnames first. Document when macOS can change the hostname without Tailscale causing it, include the cited explanation and prevention commands, and confirm whether the guidance belongs in the KB.

Written by the indexing model from the issue text.