Suggest using `-> impl Trait` when returning type parameter `-> T where T: Trait`

[estebank]

Some newcomers are confused by impl Trait in return position because they have first internalized impl Trait as an alias for type parameters in argument position. Because of this, some people try the following:

trait Trait {}
impl Trait for () {}

// this works
fn foo() -> impl Trait {
    ()
}

// this doesn't
fn bar<T: Trait>() -> T {
    ()
}

We don't currently point them in the right direction, like we do in other cases.

Took inspiration from https://twitter.com/JaapRood/status/1400782159812628482

[Noratrieb]

@rustbot claim