hPutStrLn is not atomic

Since the newline is added in an additional hPut call another thread might write to the handle in the meantime.

I think that's acceptable but should be documented.

```
-- | Write a ByteString to a handle, appending a newline byte
hPutStrLn :: Handle -> ByteString -> IO ()
hPutStrLn h ps
| length ps < 1024 = hPut h (ps `snoc` 0x0a)
| otherwise = hPut h ps >> hPut h (singleton (0x0a)) -- don't copy
```