nixos/manual: Add chapter about instance unit overrides

Hence [garbage collection](#sec-nix-gc) will remove that file and you

will wind up with a broken symlink in your systemd configuration, which

in turn will not make the service / timer start on login.

## Template units {#sect-nixos-systemd-template-units}

systemd supports templated units where a base unit can be started multiple

times with a different parameter. The syntax to accomplish this is

`service-name@instance-name.service`. Units get the instance name passed to

them (see `systemd.unit(5)`). NixOS has support for these kinds of units and

for template-specific overrides. A service needs to be defined twice, once

for the base unit and once for the instance. All instances must include

`overrideStrategy = "asDropin"` for the change detection to work. This

example illustrates this:

```nix

{

  systemd.services = {

    "base-unit@".serviceConfig = {

      ExecStart = "...";

      User = "...";

    };

    "base-unit@instance-a" = {

      overrideStrategy = "asDropin"; # needed for templates to work

      wantedBy = [ "multi-user.target" ]; # causes NixOS to manage the instance

    };

    "base-unit@instance-b" = {

      overrideStrategy = "asDropin"; # needed for templates to work

      wantedBy = [ "multi-user.target" ]; # causes NixOS to manage the instance

      serviceConfig.User = "root"; # also override something for this specific instance

    };

  };

}

```