[Tarantool-patches] [PATCH 2/2] tuple: make box.tuple.is() public

[Vladislav Shpilevoy]

Hi!

Ok, changed back to box.tuple.is() doc request.
New commit message:

	tuple: document all missed box.tuple.* methods

	In #4684 it was found that box.tuple.* contained some private
	functions: bless(), encode(), and is().

	Bless() and encode() didn't make any sense for a user, so they
	were hidden into box.internal.tuple.*.

	But box.tuple.is() is actually a useful thing. It is harnessed in
	the tests a lot, and is likely to be already used by customers,
	because it is available in box.tuple.* for a long time. It is a
	matter of time when someone will open a doc ticket saying that
	box.tuple.is() is not documented. The patch makes it legally
	public.

	Follow-up #4684

	@TarantoolBot document
	Title: box.tuple.is()
	```Lua
	box.tuple.is(object)
	```
	A function to check whether a given object is a tuple cdata
	object. Returns true or false. Never raises nor returns an error.