Document msgpack object iterator take_array method

[TarantoolBot]

Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/msgpack/#msgpack-object-methods

The new method copies the given number of msgpack values starting from
the iterator cursor position to a new msgpack array object. On success
it returns the new msgpack object and advances the iterator cursor. If
there isn't enough values to decode, it raises a Lua error and leaves
the iterator cursor unchanged.

This function could be implemented in Lua like this:

function take_array(iter, count)
    local array = {}
    for _ = 1, count do
        table.insert(array, iter:take())
    end
    return msgpack.object(array)
end

Usage example:

local mp = msgpack.object({10, 20, 30, 40})
local it = mp:iterator()
it:decode_array_header()       -- returns 4
it:decode()                    -- returns 10
local mp2 = it:take_array(2)
mp2:decode()                   -- returns {20, 30}
it:decode()                    -- returns 40

Requested by @locker in tarantool/tarantool@68bf388.