[2pt] Document the new "takes_raw_args" function option

[TarantoolBot]

Product: Tarantool
Since: 2.10.0-beta2
Root document:

• https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_schema/func_create/
• https://www.tarantool.io/ru/doc/latest/reference/reference_lua/msgpack/

SME: @ locker

Details

A new function option was added - takes_raw_args:

box.schema.func.create('my_func', {takes_raw_args = true})

If set for a Lua function, the functions arguments will be passed
wrapped in a msgpack object (see msgpack.object()) when the function
is called over IPROTO or by box.func.NAME:call():

local msgpack = require('msgpack')
local my_func = function(mp)
    assert(msgpack.is_object(mp))
    local args = mp:decode() -- array of arguments
end

Using this option might improve performance in case a function forwards
most of its arguments to another instance or writes them to a database,
because it eliminates msgpack decoding in Lua.
Requested by @locker in tarantool/tarantool@d2a0124

---

Additional comment by @ locker:
The option can be used in any custom function registered in box.schema. If such a function is called via net.box (conn:call) or box.func.call, arguments will be passed wrapped in a msgpack object.