CSV parsing: in call of get($R, $Z), what if $R is out of range

[michaelhkay]

In the description of the ?get($R, $Z) callback function, it is stated what happens if $Z doesn't identify an actual column (error when $Z is a string, return "" when it is an integer), but it's not stated what happens when $R (the row number) is out of range.

The existing text could also be improved because it talks of "the argument" to the function, when there are two.

The Saxon implementation currently returns "" and I proposed to say that unless anyone objects.