Auto merge of #114855 - Urgau:rustdoc-typedef-inner-variants, r=Guill…

…aumeGomez

rustdoc: show inner enum and struct in type definition for concrete type

This PR implements the [Display enum variants for generic enum in type def page](https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/Display.20enum.20variants.20for.20generic.20enum.20in.20type.20def.20page) #rustdoc/zulip proposal.

This proposal comes from looking at [`TyKind`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/type.TyKind.html) typedef from the compiler. On that page, the documentation is able to show the layout for each variant, but not the variants themselves. This proposal suggests showing the fields and variants for those "concrete type". This would mean that instead of having many unresolved generics, like in `IrTyKind`:
```rust
    Array(I::Ty, I::Const),
    Slice(I::Ty),
    RawPtr(I::TypeAndMut),
    Ref(I::Region, I::Ty, I::Mutability),
    FnDef(I::DefId, I::GenericArgsRef),
```
those would be resolved with direct links to the proper types in the `TyKind` typedef page:
```rust
    Array(Ty<'tcx>, Const<'tcx>),
    Slice(Ty<'tcx>),
    RawPtr(TypeAndMut<'tcx>),
    Ref(Region<'tcx>, Ty<'tcx>, Mutability<'tcx>),
    FnDef(DefId<'tcx>, GenericArgsRef<'tcx>),
```
Saving both time and confusion.

-----

<details>

<summary>Old description</summary>

I've chosen to add the enums and structs under the "Show Aliased Type" details, as well as showing the variants and fields under the usual "Variants" and "Fields" sections. ~~*under new the `Inner Variants` and `Inner Fields` sections (except for their names, they are identical to the one found in the enum, struct and union pages). Those sections are complementary and do not replace anything else.*~~

This PR proposes the following condition for showing the aliased type (basically, has the aliased type some generics that are all of them resolved):
 - the typedef does NOT have any generics (modulo lifetimes)
 - AND the aliased type has some generics

</details>

### Examples

```rust
pub enum IrTyKind<'a, I: Interner> {
    /// Doc comment for AdtKind
    AdtKind(&'a I::Adt),
    /// and another one for TyKind
    TyKind(I::Adt, I::Ty),
    // no comment
    StructKind { a: I::Adt, },
}

pub type TyKind<'a> = IrTyKind<'a, TyCtxt>;
```
![TyKind](https://github.com/rust-lang/rust/assets/3616612/13307679-6d48-40d6-ad50-6db0b7f36ac7)

<details>
<summary>Old</summary>

![image](https://github.com/rust-lang/rust/assets/3616612/4147c049-d056-42d4-8a01-d43ebe747308)

![TyKind](https://user-images.githubusercontent.com/3616612/260988247-34831aa9-470d-4286-ad9f-3e8002153a92.png)

![TyKind](https://github.com/rust-lang/rust/assets/3616612/62381bb3-fa0f-4b05-926d-77759cf9115a)

</details>

```rust
pub struct One<T> {
    pub val: T,
    #[doc(hidden)]
    pub inner_tag: u64,
    __hidden: T,
}

/// `One` with `u64` as payload
pub type OneU64 = One<u64>;
```
![OneU64](https://github.com/rust-lang/rust/assets/3616612/d551b474-ce88-4f8c-bc94-5c88aba51424)

<details>
<summary>Old</summary>

![image](https://github.com/rust-lang/rust/assets/3616612/1a3f53c0-17bf-4aa7-894d-3fedc15b33da)

![OneU64](https://github.com/rust-lang/rust/assets/3616612/7b124a5b-e287-4efb-b9ca-fdcd1cdeeba8)

![OneU64](https://github.com/rust-lang/rust/assets/3616612/ddd962be-4f76-4ecd-81bd-531f3dd23832)

</details>

r? `@GuillaumeGomez`