Give references about self_referencing in 1.2 Merge Iterator

[ppdogg]

It took me quite a while to know how MemTableIterator::new is generated and the reason to pass a closeur/fn as parameter

Since in the text, you've already mentioned ouroboros, I think it is reasonable and understandable to provide the link below:

https://docs.rs/ouroboros/latest/ouroboros/attr.self_referencing.html

[skyzh]

Thanks for the suggestion :) I'll update the book