nixos/tools: add enable options to manual

E.g. when overriding `nix` with `pkgs.lix`, `nixos-option` will fail.
Given that I haven't used it in a very long time, I wanted to disable
it, but finding an option to turn off `nixos-option` is kinda hard given
that the options are generated here using `mkToolModule`.

I assumed that this isn't possible until I learned that
`system.tools.X.enable` exists. To me, this is a clear sign that these
shouldn't be internal.

(cherry picked from commit 859c76c505)

nixos/modules/installer/tools/tools.nix

@@ -218,7 +218,7 @@ in
     mkToolModule = { name, package ? pkgs.${name} }: { config, ... }: {
       options.system.tools.${name}.enable = lib.mkEnableOption "${name} script" // {
         default = config.nix.enable && ! config.system.disableInstallerTools;
-        internal = true;
+        defaultText = "config.nix.enable && !config.system.disableInstallerTools";
       };
 
       config = lib.mkIf config.system.tools.${name}.enable {