docs(gateway): describe what is, not what changed

Per review: docs represent current state. Every "used to" / "no longer"
clause this branch introduced is gone — including the History section in
network.md, which was a whole subsection about a sync mechanism that
doesn't exist.

Where the removed clause was carrying a real constraint, the constraint
stays and is stated in the present tense instead of as a delta: nothing
narrows what the gateway's nginx can reach except the directory
permissions in front of a socket, and nothing bounds `ReloadGatewayNginx`
except the hard-coded unit name. Those read as rules now rather than as
the story of how they came to be rules.

swarm-controller/README.md

@@ -42,11 +42,9 @@ What keeps that safe is that the directory holds **one** socket. So:
 > Pointing nginx at that directory to reach this socket would put the admin
 > socket within its reach too.
 
-This got *less* forgiving when nginx moved onto the host: the gateway used to
-reach a unix upstream through a bind-mount, so the mount list was a second
-bound on what it could touch. There is no mount now — the directory is the
-whole of the access control. A unit test pins the default path so a tidying
-edit fails instead of reviewing cleanly.
+nginx is a host service, so nothing narrows what it can reach except the
+directory itself — that is the whole of the access control. A unit test pins
+the default path so a tidying edit fails instead of reviewing cleanly.
 
 `RuntimeDirectoryPreserve=yes` and the daemon's stale-socket unlink on start are
 a **pair**: preserving the directory without the unlink means `bind` fails with