Add comment block to `system/packages.nix`

This is meant as documentation in case I want to enable `documentation.{man,nixos}` in the future, though I honestly doubt that I will. These would fix tools like `whatis`, but apparently `whatis` only seems to work on packages installed via `environment.systemPackages` that have man pages.

system/packages.nix

@@ -9,7 +9,18 @@
     */
     # agenix.packages.x86_64-linux.default
   ];
-  
+
+  /*
+  documentation = {
+    nixos = {
+      includeAllModules = true;
+    };
+    man = {
+      generateCaches = true;
+    };
+  };
+  */
+
   nixpkgs = {
     config = {
       allowUnfree = true;