Stabilize rwlock_downgrade library feature

[connortsui20]

Tracking Issue: #128203

Method to be stabilized:

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: #128203 (comment)

[rustbot]

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[Amanieu]

This means that we can never go back to pthreads to back our locks, but that seems fine since the pthreads API has many issues around UB and movability.

@rfcbot merge

[Amanieu]

@rfcbot merge

[rfcbot]

Team member @Amanieu has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

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