argus on #661🟡: "matrix IDs embed server_name irrevocably — anyone
already running with the old default would be broken."
Append a `**Breaking change as of #660**` paragraph to the
`serverName` option description with the exact opt-back-in string,
matching the pattern from #651's openFirewall flip. PR body + commit
already documented the breakage; this surfaces it in the option's
own description so it shows up in `nix flake show` + the auto-
generated options docs right next to the option.
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