nixos/*nix*: Update module impl docs and link related modules

Something extra for the readers.

nixos/modules/config/flakes.nix

@@ -1,3 +1,10 @@
+/*
+  Manages the flake registry.
+
+  See also
+   - ./nix.nix
+   - ./nix-channel.nix
+ */
 { config, lib, ... }:
 let
   inherit (lib)

nixos/modules/config/nix-channel.nix

@@ -1,3 +1,11 @@
+/*
+  Manages the things that are needed for a traditional nix-channel based
+  configuration to work.
+
+  See also
+   - ./nix.nix
+   - ./flakes.nix
+ */
 { config, lib, ... }:
 let
   inherit (lib)

nixos/modules/config/nix-remote-build.nix

@@ -1,3 +1,10 @@
+/*
+  Manages the remote build configuration, /etc/nix/machines
+
+  See also
+   - ./nix.nix
+   - nixos/modules/services/system/nix-daemon.nix
+ */
 { config, lib, ... }:
 
 let

nixos/modules/config/nix.nix

@@ -1,5 +1,11 @@
 /*
-  Manages /etc/nix.conf, build machines and any nix-specific global config files.
+  Manages /etc/nix.conf.
+
+  See also
+   - ./nix-channel.nix
+   - ./flakes.nix
+   - ./nix-remote-build.nix
+   - nixos/modules/services/system/nix-daemon.nix
  */
 { config, lib, pkgs, ... }:
 

nixos/modules/services/system/nix-daemon.nix

@@ -1,3 +1,10 @@
+/*
+  Declares what makes the nix-daemon work on systemd.
+
+  See also
+   - nixos/modules/config/nix.nix: the nix.conf
+   - nixos/modules/config/nix-remote-build.nix: the nix.conf
+*/
 { config, lib, pkgs, ... }:
 
 with lib;