fix documentation for resultErrorMessage

previously it had the doc string for `errorMessage`

Author: bergey

src/Database/PostgreSQL/LibPQ.hsc

@@ -1045,8 +1045,8 @@ resStatus es =
        return $ B.fromForeignPtr fp 0 $ fromIntegral len
 
 
--- | Returns the error message most recently generated by an operation
--- on the connection.
+-- | Returns the error message associated with the command, or an
+-- empty string if there was no error.
 resultErrorMessage :: Result
                    -> IO (Maybe B.ByteString)
 resultErrorMessage = flip maybeBsFromResult c_PQresultErrorMessage