Rollup merge of #80805 - camelid:iter-by_ref-example, r=steveklabnik

Improve `Iterator::by_ref` example

I split the example into two: one that fails to compile, and one that
works. I also made them identical except for the addition of `by_ref`
so we don't confuse readers with random differences.

cc `@steveklabnik,` who is the one that added the previous version of this example

diff --cc library/core/src/iter/traits/iterator.rs
Simple merge