passwordFile: update description

I updated the description of passwordFile because if you do not define a full path to the passwordFile you can easily lock out yourself from the system.

e.g.:

configuration.nix is in /etc/nixos/configuration.nix
password file is in /etc/nixos/pwd.root

mutableUsers=false;
passwordFile="./pwd.root";

nixos-rebuild does it's job because it can find the path defined in passwordFile but on next reboot the file isn't accessible. This took me a while to figure out because my SSD is booting so fast that i couldn't read the error message thast the file couldn't be found.