rustdoc: support RTN once its syntax is somewhat finalized

[fmease]

This is blocked by T-lang's decision on the final syntax of RTN!

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Source:

#![feature(return_type_notation)]

pub fn f<T: Trait<f(): Copy>>() {}

pub trait Trait { fn f() -> impl Clone; }

Current rendered output of f (local scenario, HIR-based cleaning):

pub fn f<T: Trait<f: Copy>>()

Current rendered output of f (inlined cross-crate re-export scenario, rustc_middle::ty-based cleaning):

pub fn f<T>()
where
    T: Trait,
    impl Clone: Copy,