[rust/en] clarify the difference between `impl<T> Foo<T>` and `impl Foo<T>`

This was confusing to me. At first glance, the first `T` just looks redundant because `Foo` is already
polymorphic. I'd add a note saying that the first `T` is necessary to indicate that `T` in `Foo` here
is not a concrete type, but a type parameter. Or, perhaps add an example where `T` is concrete, then `impl Foo` will get accepted.