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/otel.nix

@@ -68,7 +68,7 @@ in
       ⚠️ The collector is therefore in the path of all telemetry. It runs
       on the same host as the agents and restarts on failure, and
       telemetry is not the control plane, so degraded telemetry is not
-      degraded operation — but the export no longer survives independently
+      degraded operation — but the export does not survive independently
       of anything host-side
     '';
 
@@ -239,7 +239,7 @@ in
         onward, which is one hop for the whole hive rather than one per
         target.
 
-        No longer empty on a default hive: this collector's own endpoint
+        Not empty on a default hive: this collector's own endpoint
         is always in here, so the scrape receiver is emitted everywhere.
         The `!= { }` guards around it stay meaningful — the set can still
         be forced empty, and a `prometheus` receiver with nothing to