Example could use a for loop to show processing results, clarify Results

[tv42]

rust-analyzer thinks the example's store.search return type is <&str as Searcher<Book>>::Item, which is not very useful. Store::search docs are not great either, leaving it unclear what I::Item is.

An extra source of confusion is that Item sounds like an item of an iterator, where in this case it apparently refers to results, plural.

It took a couple of tries, e.g. attempting to iterate the return value directly, before I stumbled on Results and saw how to iterate the results correctly.

It's not very clear how one would navigate from Store::search to Results based purely on the docs.