argus picked option (a) on #653: put the upgrade note in each option's
`description` so it shows up in `nix flake show` + the rendered
options docs, right next to the option itself. cheapest option, no
eval-time noise (a `warnings` block would fire on every new
deployment that wants false — the normal case now).
Appended a `**Breaking change as of #651**` paragraph to each of the
three `openFirewall` descriptions, naming the exact option string the
operator needs to set to restore the old behaviour.
Gateway's note specifically calls out that external reach is the
common case (operator's primary entry point), so the upgrade hint
is most likely needed there.
85790b0cca