`socket.tcp_server_create()` and `socket.tcp_server_loop()`

socket.tcp_server_create() creates a master socket, performs bind
and listen, and applies the optional prepare callback. It returns
the listening socket and its address.

socket.tcp_server_loop() runs the accept loop for an existing
listening socket and calls the user handler for each connection.

Usage example:

local socket = require('socket')
local fiber = require('fiber')

local function prepare(s)
    s:setsockopt('SOL_SOCKET', 'SO_REUSEADDR', true)
    return 128
end

local function handler(sc)
    sc:write('hello')
    sc:close()
end

local s, addr = socket.tcp_server_create('127.0.0.1', 12340, prepare)
fiber.create(socket.tcp_server_loop, s, handler)

legacy single-call API still works:

local socket = require('socket')

local function prepare(s)
    s:setsockopt('SOL_SOCKET', 'SO_REUSEADDR', true)
    return 128
end

local function handler(sc)
    sc:write('hello')
    sc:close()
end

socket.tcp_server('127.0.0.1', 12340, {prepare=prepare, handler=handler})

Requested by @unera in https://github.com/tarantool/tarantool/commit/1408f36701444b4fa4a7f9f67dee9438770a6bed.

The issue describes socket.tcp_server_create(), socket.tcp_server_loop(), and the legacy socket.tcp_server() API with Lua usage examples. Start by locating the socket API documentation and the existing tcp_server() entry; document the two split APIs, their callbacks and return values, while preserving the legacy example.