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/swarm-victorialogs.nix

@@ -262,6 +262,5 @@ in
   # ⚠️ Like the metrics store's, that endpoint is unauthenticated — which is
   # why `listenAddress` above is loopback and why nothing reaches it except
   # through the gateway, where the ingest route is authenticated. (This module
-  # does declare a vhost; the line that used to say otherwise was already
-  # wrong before the collector started pushing through it.)
+  # declares a vhost.)
 }