Incomplete space/index iterator documentation

[remagister]

Motivation
Methods space.pairs and index.pairs return an iterator that might be used with for-in loop, as stated in documentation. However there is no information about the structure and origin of the provided iterator (in both en and ru versions).

Suggestion
Please insert links to the luafun documentation, that describe iterator structure, iterator_mt metatable functions or at least mention that luafun methods are available to use.

[Totktonada]

@pgulutzan I hope a user want to know about methods (s)he can use on this iterator object, not about luafun internals. I think that a sentence that :pairs() returns luafun iterator and the link to the luafun documentation ( https://luafun.github.io/index.html ) is enough.

[pgulutzan]

So what's the issue here?
You're objecting that the manual should not point to https://luafun.github.io/under_the_hood.html
and saying it should point to https://luafun.github.io/index.html?

[Totktonada]

Yep, I think so.

[pgulutzan]

Okay.

[Totktonada]

For information about iterators’ internal structures see the [“Lua Functional library”(https://luafun.github.io/index.html) documentation.

We again meet the situation, when a user highlights that he is unable to find a luafun method in the documentation. I propose:

1. Change the link description. It is not about internal structures, it is about user's API.
2. Add a table with luafun methods to at least make it searchable.

The pages of the topic:

https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/pairs/
https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_index/pairs/