lock_api: `RwLockReadGuard::unlocked` incorrectly claims fairness

[Christopher Durham]

RwLockReadGuard::unlocked:

parking_lot/lock_api/src/rwlock.rs

Lines 1248 to 1265 in de2a402

 /// Temporarily unlocks the `RwLock` to execute the given function. 
 /// 
 /// The `RwLock` is unlocked a fair unlock protocol. 
 /// 
 /// This is safe because `&mut` guarantees that there exist no other 
 /// references to the data protected by the `RwLock`. 
 #[inline] 
 pub fn unlocked<F, U>(s: &mut Self, f: F) -> U 
 where 
     F: FnOnce() -> U, 
 { 
     // Safety: An RwLockReadGuard always holds a shared lock. 
     unsafe { 
         s.rwlock.raw.unlock_shared(); 
     } 
     defer!(s.rwlock.raw.lock_shared()); 
     f() 
 } 

RwLockReadGuard::unlocked_fair:

parking_lot/lock_api/src/rwlock.rs

Lines 1290 to 1307 in de2a402

 /// Temporarily unlocks the `RwLock` to execute the given function. 
 /// 
 /// The `RwLock` is unlocked a fair unlock protocol. 
 /// 
 /// This is safe because `&mut` guarantees that there exist no other 
 /// references to the data protected by the `RwLock`. 
 #[inline] 
 pub fn unlocked_fair<F, U>(s: &mut Self, f: F) -> U 
 where 
     F: FnOnce() -> U, 
 { 
     // Safety: An RwLockReadGuard always holds a shared lock. 
     unsafe { 
         s.rwlock.raw.unlock_shared_fair(); 
     } 
     defer!(s.rwlock.raw.lock_shared()); 
     f() 
 } 

Fix: just remove the line about being fair from RwLockReadGuard::unlocked's documentation.

This mistake is not present on upgradable read guards, write guards, nor mutex guards.