Add docs on how to create a VPN client private and pubkey in one line

nixos/modules/monitoring/vpn/client.nix

@@ -12,6 +12,8 @@ in {
       default = /run/keys/monitoringvpn/client.key;
       description = ''
         File with base64 private key generated by <command>wg genkey</command>.
+        Shorthand to create private and public key:
+        <command>wg genkey | tee peer_A.key | wg pubkey > peer_A.pub</command>
       '';
     };
     presharedKeyFile = lib.mkOption {