I admit that I do not know. All classic TA functions as well as their descriptions are from the standard technical indicator library, the TA-Lib that is used by most trading platforms. But I believe that the description is correct, because using close[-1] would peek into the future and thus make no sense. You can easily test this by comparing the TR return value with max(high[0],close[0]) - min(low[0],close[0]).