Fix doc example to make it clear that ".returning()" does not preserve original order

[greyblake]

The current doc example implies, that the order of returned values are always preserved:

let inserted_names = diesel::insert_into(users)
    .values(&vec![name.eq("Timmy"), name.eq("Jimmy")])
    .returning(name)
    .get_results(&connection);
assert_eq!(Ok(vec!["Timmy".to_string(), "Jimmy".to_string()]), inserted_names);

But unfortunately it's not guaranteed (at least definitely not by PostgreSQL DB, which is used for this doc test).

See more: https://stackoverflow.com/questions/5439293/is-insert-returning-guaranteed-to-return-things-in-the-right-order

P.S.
Thank you for the nice and reliable piece of software ❤️

[weiznich]

Thanks for opening this PR and improving the documentation 👍

(Tests will likely fail due to chronotope/chrono#1389, I will take care of that later on)

[greyblake]

@weiznich Thank you!