Rollup merge of #114059 - fmease:rustdoc-fix-x-crate-impl-sized, r=GuillaumeGomez

rustdoc: fix cross-crate `impl Sized` & `impl ?Sized`

Previously, cross-crate impl-Trait (APIT, RPIT, etc.) that only consists of a single `Sized` bound (modulo outlives-bounds) and ones that are `?Sized` were incorrectly rendered. To give you a taste (before vs. after):

```diff
- fn sized(x: impl ) -> impl
+ fn sized(x: impl Sized) -> impl Sized

- fn sized_outlives<'a>(x: impl 'a) -> impl 'a
+ fn sized_outlives<'a>(x: impl Sized + 'a) -> impl Sized + 'a

- fn maybe_sized(x: &impl ) -> &impl
+ fn maybe_sized(x: &impl ?Sized) -> &impl ?Sized

- fn debug_maybe_sized(x: &impl Debug) -> &impl ?Sized + Debug
+ fn debug_maybe_sized(x: &(impl Debug + ?Sized)) -> &(impl Debug + ?Sized)
```

Moreover, we now surround impl-Trait that has multiple bounds with parentheses if they're the pointee of a reference or raw pointer type. This affects both local and cross-crate docs. The current output isn't correct (rustc would emit the error *ambiguous `+` in a type* if we fed the rendered code back to it).

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Best reviewed commit by commit :)

`@rustbot` label A-cross-crate-reexports

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