docs: mention that killThread on a completed thread is a no-op

diff --git a/GHC/Conc.lhs b/GHC/Conc.lhs
index 6239278..b19f520 100644 (file)
--- a/GHC/Conc.lhs
+++ b/GHC/Conc.lhs
@@ -266,6 +266,7 @@ terms of 'throwTo':
 
 > killThread tid = throwTo tid ThreadKilled
 
+Killthread is a no-op if the target thread has already completed.
 -}
 killThread :: ThreadId -> IO ()
 killThread tid = throwTo tid ThreadKilled