Use impl IntoResponse less in docs

[davidpdrsn]

Previously we used impl IntoResponse quite a lot in the docs to avoid having to write the full return type. However we've seen several cases on Discord and Twitter where people who are new to Rust gets confused about that. People sometimes thinks it allows returning different types along the lines of

async fn handle() -> impl IntoResponse {
    if something() {
        "foo"
    } else {
        StatusCode::BAD_REQUEST
    }
}

Which unfortunately wont work 😕 Writing out the full type should make the issue clear.

So I think we're actually doing people a disservice using impl IntoResponse as much as we do. In general I think our docs should cater to beginners perhaps more than other crates, since web frameworks are likely to be the first big crate a newcomer picks up.

So this changes the docs to spell out the return type where it is appropriate. I also added a section about calling .into_response() to more easily return different types.

[bsodmike]

@davidpdrsn doesn't the Impl IntoReponse interface recurse over the tuple essentially calling .into_reponse() on each tuple element? Referring to all_the_tuples!(impl_into_response); in the source of the trait.

[davidpdrsn]

It does yes.