nixos/pulseaudio: document audio group for system wide access

nixos/modules/config/pulseaudio.nix

···
           each user that tries to use the sound system. The server runs
           with user privileges. This is the recommended and most secure
           way to use PulseAudio. If true, one system-wide PulseAudio
-          server is launched on boot, running as the user "pulse".
+          server is launched on boot, running as the user "pulse", and
+          only users in the "audio" group will have access to the server.
           Please read the PulseAudio documentation for more details.
         '';
       };