doc: add code comment clarifying the meaning of legacyPackages

flake.nix

@@ -45,6 +45,15 @@
         }).nixos.manual.x86_64-linux;
       };
 
+      # The "legacy" in `legacyPackages` doesn't imply that the packages exposed
+      # through this attribute are "legacy" packages. Instead, `legacyPackages`
+      # is used here as a substitute attribute name for `packages`. The problem
+      # with `packages` is that it makes operations like `nix flake show
+      # nixpkgs` unusably slow due to the sheer number of packages the Nix CLI
+      # needs to iterate through. But when the Nix CLI sees a `legacyPackages`
+      # attribute it displays `omitted` instead of iterating through all
+      # packages, which keeps `nix flake show` on Nixpkgs reasonably fast,
+      # though less information rich.
       legacyPackages = forAllSystems (system: import ./. { inherit system; });
 
       nixosModules = {