nixos/rtkit: mention pipewire in docstring

I don't know the reason for rtkit only getting enabled by
hardware.pulseaudio.enable and not services.pipewire.enable, as they
both use it to get real-time priority, but we can at least help users by
mentioning pipewire in the rtkit option.

nixos/modules/security/rtkit.nix

@@ -15,7 +15,7 @@ with lib;
       description = ''
         Whether to enable the RealtimeKit system service, which hands
         out realtime scheduling priority to user processes on
-        demand. For example, the PulseAudio server uses this to
+        demand. For example, PulseAudio and PipeWire use this to
         acquire realtime priority.
       '';
     };