doc: add code comment clarifying the meaning of legacyPackages

        }).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 = {