rustdoc: Add support for `#[rustc_must_implement_one_of]`
This PR adds support for `#[rustc_must_implement_one_of]` attribute added in #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>