…ort_for_must_implement, r=GuillaumeGomez

rustdoc: Add support for `#[rustc_must_implement_one_of]`

This PR adds support for `#[rustc_must_implement_one_of]` attribute added in rust-lang#92164. There is a desire to eventually use this attribute of `Read`, so making it show up in docs is a good thing.

I "stole" the styling from cfg notes, not sure what would be a proper styling. Currently it looks like this:
![2022-07-14_15-00](https://user-images.githubusercontent.com/38225716/178968170-913c1dd5-8875-4a95-9848-b075a0bb8998.png)

<details><summary>Code to reproduce</summary>
<p>

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

#[rustc_must_implement_one_of(a, b)]
pub trait Trait {
    fn req();
    fn a(){ Self::b() }
    fn b(){ Self::a() }
}
```

</p>
</details>

remove useless mut from examples

remove useless mut from examples

…Dylan-DPC

Fix some broken link fragments.

An exception for link fragments starting with `-` was added in rust-lang#49590. However, it is not clear what issues were encountered at the time. Perhaps those were fixed in the meantime.

This removes the exception, and fixes a couple of broken links that were skipped due to it.

…woerister

Remove unused tuple fields

Found by rust-lang#95977

Make `transmute_copy` docs read better

…r=Dylan-DPC

remove useless `#[allow]` in a test

The mentioned issue, rust-lang#54586 was fixed 4 years ago :)