code: func RPCAuthFlag(f *flag.FlagSet) *string {
  	rpcAuth := os.Getenv("SERF_RPC_AUTH")
  	return f.String("rpc-auth", rpcAuth,
  		"RPC auth token of the Serf agent")
  }
docs: // RPCAuthFlag returns a pointer to a string that will be populated
  // when the given flagset is parsed with the RPC auth token of the Serf.