…urther-specializable, r=BoxyUwU

Still-further-specializable projections are ambiguous in new solver

Fixes https://github.com/rust-lang/rust/pull/108896/files#r1148450781

r? `@BoxyUwU` (though feel free to re-roll)

---

This can be used to create an unsound transmute function with the new solver:

```rust
#![feature(specialization)]

trait Default {
   type Id;

   fn intu(&self) -> &Self::Id;
}

impl<T> Default for T {
   default type Id = T;

   fn intu(&self) -> &Self::Id {
        self
   }
}

fn transmute<T: Default<Id = U>, U: Copy>(t: T) -> U {
    *t.intu()
}

use std::num::NonZeroU8;
fn main() {
    let s = transmute::<u8, Option<NonZeroU8>>(0);
    assert_eq!(s, None);
}
```

…iaskrgr

Rollup of 6 pull requests

Successful merges:

 - rust-lang#109007 (rustdoc: skip `// some variants omitted` if enum is `#[non_exhaustive]`)
 - rust-lang#109593 (Rustdoc Book refer to rustdoc::missing_doc_code_examples. Fixes rust-lang#109592.)
 - rust-lang#109595 (Improve "Auto-hide trait implementation documentation" GUI test)
 - rust-lang#109619 (Still-further-specializable projections are ambiguous in new solver)
 - rust-lang#109620 (Correct typo (`back_box` -> `black_box`))
 - rust-lang#109621 (Refactor: `VariantIdx::from_u32(0)` -> `FIRST_VARIANT`)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup