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/packages/default.nix

@@ -61,10 +61,10 @@ let
   # derivation means a prompt edit doesn't bust the cargo cache.
   #
   # …and the dep cache underneath is the one WITHOUT test targets. This
-  # derivation has never run a test — `doCheck = false` below says so —
-  # but it used to sit on a cache that compiled every dev-dependency and
-  # test harness anyway, work whose only consumers are the checks. A
-  # deploy paid for it on every toolchain or dependency change.
+  # derivation never runs a test — `doCheck = false` below says so — so it
+  # sits on a cache that skips every dev-dependency and test harness, work
+  # whose only consumers are the checks. Compiling them here would make a
+  # deploy pay for it on every toolchain or dependency change.
   workspaceBuild = craneLib.buildPackage {
     src = cleanSrc;
     cargoArtifacts = cargoArtifactsBinOnly;