* manual: Mark commands that require root
Mark every command that requires to be run as root by prefixing them
with '#' instead of '$'.
* manual: Add note about commands that require root
Having configuration.nix generation hidden underneath nixos-option
never made sense, also given that there was another command to
generate part of the configuration (nixos-hardware-scan). Now
nixos-generate-config produces both configuration.nix and
hardware-configuration.nix. The latter is overwritten while the
former is not.
