This adds a NixOS option for setting the CPU max and min frequencies with `cpufreq`. The two options that have been added are: - `powerManagement.cpufreq.max` - `powerManagement.cpufreq.min` It also adds an alias to the `powerManagement.cpuFreqGovernor` option as `powerManagement.cpufreq.governor`. This updates the installer to use the new option name. It also updates the manual with a note about the new name.
To build the manual, you need Nix installed on your system (no need
for NixOS). To install Nix, follow the instructions at
    https://nixos.org/nix/download.html
When you have Nix on your system, in the root directory of the project
(i.e., `nixpkgs`), run:
    nix-build nixos/release.nix -A manual.x86_64-linux
When this command successfully finishes, it will tell you where the
manual got generated.