Rollup merge of #143191 - connortsui20:stabilize-rwlock-downgrade, r=tgross35

Stabilize `rwlock_downgrade` library feature

Tracking Issue: rust-lang/rust#128203

Method to be stabilized:

```rust
impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
    pub fn downgrade(s: Self) -> RwLockReadGuard<'a, T> {}
}
```

~~I would like to point out that my documentation comment is longer than ideal, but at the same time I don't really know how else to show why `downgrade` is actually necessary (instead of just unlocking and relocking). If anyone has ideas for making this more concise that would be great!~~ I have made the documentation a bit more clear.

Stabilization report: rust-lang/rust#128203 (comment)

Author: matthiaskrgr