docs(3191): drop the migration history from the gateway comments

Per mara on the PR: how a thing used to be deployed stops being
relevant the moment this is merged and the hives are rebuilt, so the
comments state the current constraint and nothing about the container
that used to hold it.

Removes the two ⚠️ blocks this PR added (hive-tls-ca's ordering, the
resign propagation), the matrix ordering's, dnsmasq's resolv.conf
paragraph, and the pre-existing 'REMOVED WITH THE CONTAINER' inventory
in the gateway module. The 🚨 do-not-simplify warning on the cert-copy
unit keeps both its reasons and loses the container framing.

Kept deliberately: the two operator-facing 'this used to default to
true' option notes (migration information for someone upgrading), and
the SupplementaryGroups block, which documents why an absence is
load-bearing rather than how a deployment used to look.

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

@@ -69,11 +69,8 @@
     dhcp-leasefile = "/var/lib/dnsmasq/dnsmasq.leases";
     # No explicit upstream: non-hive queries follow dnsmasq's
     # resolv.conf default — the host's own `/etc/resolv.conf`, so the
-    # hive always uses the host's resolvers and follows them live. This
-    # used to be a *copy* nixos-container made at container start, kept
-    # fresh by a host-side path unit that pushed in a new one and
-    # reloaded dnsmasq; running on the host deleted both the copy and
-    # the machinery that watched it. Deliberately no fallback
+    # hive always uses the host's resolvers and follows them live with
+    # no copy to go stale. Deliberately no fallback
     # `server=`: dnsmasq queries
     # all known upstreams in parallel, so a hardcoded public resolver
     # would take a share of *normal* traffic, not just fill in when the