clarifying iterator trait documentation
Remove misleading code example from The Guide

The removed code caused confusion because it is not clear that the type of `y` is actually `()`
auto merge of #15897 : Gankro/rust/it-docs, r=kballard

I found these things to be ambiguous, or at least worth stating explicitly to reduce the amount a user/developer needs to think about the API.
auto merge of #15899 : aochagavia/rust/guide, r=kballard

The removed code caused confusion because it is not clear that the type of `y` is actually `()`