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/module-eval/nats-authelia.nix

@@ -44,9 +44,9 @@ let
   # refuses; see ./host-modules/deploy.nix. `package` and `bridgePackage` are
   # ordinary options, so they do carry one. The two arms
   # below have different jobs. `usersFile` tests the rename; the nats one tests
-  # that a reader repointed to the new namespace still renders the derived
-  # path, which is the failure this slice could actually have shipped — seven
-  # of those reads went through an alias a path-shaped grep cannot see.
+  # that a reader pointed at the `deploy` namespace still renders the derived
+  # path — seven of those reads go through an alias a path-shaped grep
+  # cannot see.
   autheliaOldPath = hive {
     deploy.authelia.enable = true;
     deploy.nats.enable = true;
@@ -84,8 +84,8 @@ let
 
   # A hive running NOTHING of the swarm's own services — no IdP here, no
   # `swarm.authelia.url` set by hand. The whole point of the fixture is what it
-  # does *not* say: it is the shape whose IdP address used to be null, and
-  # before that a co-location-derived guess.
+  # does *not* say: no IdP address is given and none is derived from
+  # co-location.
   autheliaNotColocated = hive { };
   # The same swarm's IdP host, for the arm that the two agree.
   autheliaColocated = hive { deploy.authelia.enable = true; };