treewide: add documentation for nixos systemd units

nixos/modules/tasks/filesystems/btrfs.nix

@@ -166,6 +166,7 @@ in
             in
             nameValuePair "btrfs-scrub-${fs'}" {
               description = "btrfs scrub on ${fs}";
+              documentation = [ "man:btrfs-scrub(8)" ];
               # scrub prevents suspend2ram or proper shutdown
               conflicts = [
                 "shutdown.target"

nixos/modules/tasks/powertop.nix

@@ -51,6 +51,7 @@ in
   config = mkIf (cfg.enable) {
     systemd.services = {
       powertop = {
+        documentation = [ "man:powertop(8)" ];
         wantedBy = [ "multi-user.target" ];
         after = [ "multi-user.target" ];
         description = "Powertop tunings";