base: Add @since on GHC.IO.Handle.Lock.hUnlock

Unfortunately this was introduced in base-4.11.0 (GHC 8.4.1)
whereas the other locking primitives were added in base-4.10.0 (GHC
8.2.1).

libraries/base/GHC/IO/Handle/Lock.hsc

@@ -100,6 +100,8 @@ hTryLock :: Handle -> LockMode -> IO Bool
 hTryLock h mode = lockImpl h "hTryLock" mode False
 
 -- | Release a lock taken with 'hLock' or 'hTryLock'.
+--
+-- @since 4.11.0.0
 hUnlock :: Handle -> IO ()
 hUnlock = unlockImpl