nix: comments state current behaviour, not history

Rewrite comments and option descriptions under nix/ that narrate how the
code got here ("used to", "new name", "sat at the TOP", "no longer",
renamed/moved narration, attributions) to say what the code does now and
why. Comments and description strings only; no logic changes.

Files in open PR #4970's diff are left alone. Refs #3902

nix/host-modules/hive-gateway/options.nix

@@ -82,10 +82,6 @@ in
         nginx vhost. Leave off when running behind another reverse
         proxy (e.g. caddy / traefik on the host) that handles TLS
         termination + forwards to `port`.
-
-        **Note**: this used to default to `true`. Add
-        `services.hyperhive.gateway.openFirewall = true;` to your host
-        config if external reach stopped working after a recent upgrade.
       '';
     };