TODO: mention nixos-option and nixos.org/nixos/options.html in manual Chapter 6

[trusktr]

We should mention in nixos manual Chapter 6 that options can be dicovered with nix-option and http://nixos.org/nixos/options.html. It will help newbs learn how to discover packages more easily.

As a new user, I was sticking everything I wanted inside of the systemPackages array, and I didn't not realize that programs.foo.enable is a better route to take because it configures many things automatically. For example, programs.zsh.enable = true results in nix-zsh-completions also being installed as well as features enabled, while writing environment.systemPackages = [pkgs.zsh] does not.

This will be helpful to new users.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/programs-foo-enable-true-vs-systempackages-foo-is-confusing/5534/7