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.

hive-priv/src/main.rs

@@ -2186,11 +2186,9 @@ async fn read_container_journal(container: &str, query: &JournalQuery) -> Result
 /// - otherwise → `systemctl start nginx`
 ///
 /// ⚠️ `nginx` is hard-coded on purpose — see `PrivRequest::ReloadGatewayNginx`.
-/// The unit name is the scope of this verb, and it used to be enforced by
-/// `--machine=hive-gateway` (which could only reach into that container).
-/// With nginx on the host there is no namespace to bound it, so the
-/// literal is the only thing standing between "reload the gateway" and
-/// "reload anything".
+/// The unit name is the scope of this verb: nginx is a host unit, so no
+/// namespace bounds it and the literal is the only thing standing between
+/// "reload the gateway" and "reload anything".
 ///
 /// Returns `(String::new(), String::new())` on success so it fits the
 /// `exec` return type directly.