[docs] fix docs for machine_list_filename param

[StrikerRUS]

I might be wrong, but seems machine_list_filename parameter cannot be used via C API and in consequence by any language wrapper.
@guolinke Could you please review this PR?

LightGBM/python-package/lightgbm/basic.py

Lines 2347 to 2350 in 9eeac3c

	_safe_call(_LIB.LGBM_NetworkInit(c_str(machines),
	ctypes.c_int(local_listen_port),
	ctypes.c_int(listen_time_out),
	ctypes.c_int(num_machines)))

LightGBM/include/LightGBM/c_api.h

Lines 1266 to 1277 in 5a46084

	/*!
	* \brief Initialize the network.
	* \param machines List of machines in format 'ip1:port1,ip2:port2'
	* \param local_listen_port TCP listen port for local machines
	* \param listen_time_out Socket time-out in minutes
	* \param num_machines Total number of machines
	* \return 0 when succeed, -1 when failure happens
	*/
	LIGHTGBM_C_EXPORT int LGBM_NetworkInit(const char* machines,
	int local_listen_port,
	int listen_time_out,
	int num_machines);

LightGBM/src/c_api.cpp

Lines 2302 to 2316 in 4ae4abb

	int LGBM_NetworkInit(const char* machines,
	int local_listen_port,
	int listen_time_out,
	int num_machines) {
	API_BEGIN();
	Config config;
	config.machines = RemoveQuotationSymbol(std::string(machines));
	config.local_listen_port = local_listen_port;
	config.num_machines = num_machines;
	config.time_out = listen_time_out;
	if (num_machines > 1) {
	Network::Init(config);
	}
	API_END();
	}

LightGBM/src/network/network.cpp

Lines 30 to 32 in 9eeac3c

	void Network::Init(Config config) {
	if (config.num_machines > 1) {
	linkers_.reset(new Linkers(config));

LightGBM/src/network/linkers_socket.cpp

Lines 24 to 34 in 9eeac3c

	Linkers::Linkers(Config config) {
	is_init_ = false;
	// start up socket
	TcpSocket::Startup();
	network_time_ = std::chrono::duration<double, std::milli>(0);
	num_machines_ = config.num_machines;
	local_listen_port_ = config.local_listen_port;
	socket_timeout_ = config.time_out;
	rank_ = -1;
	// parse clients from file
	ParseMachineList(config.machines, config.machine_list_filename);

LightGBM/src/network/linkers_socket.cpp

Lines 81 to 92 in 9eeac3c

	void Linkers::ParseMachineList(const std::string& machines, const std::string& filename) {
	std::vector<std::string> lines;
	if (machines.empty()) {
	TextReader<size_t> machine_list_reader(filename.c_str(), false);
	machine_list_reader.ReadAllLines();
	if (machine_list_reader.Lines().empty()) {
	Log::Fatal("Machine list file %s doesn't exist", filename.c_str());
	}
	lines = machine_list_reader.Lines();
	} else {
	lines = Common::Split(machines.c_str(), ',');
	}

[guolinke]

yeah, the filename is for CLI only.

[jameslamb]

Thanks for your attention to detail @StrikerRUS !

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