My initial reaction was that this was either an internal field or something that had been deprecated in the RTM, yet there are references to it in the target property too. What I ultimately discovered is that this is a simple omission in the documentation.
Now back to why I was looking it up in the first place - what exactly is the difference between charCode and keyCode? Patrick Long has a blog entry at http://blogs.charteris.com/blogs/patl/archive/2007/07/04/keycodes-charcodes-and-asp-net-ajax.aspx that sums it all up nicely; charCode will return a value related to the character being pressed whereas keyCode is related to the key being pressed. For example, 7 on the number pad and 7 on the QWERTY pad will return different keyCode values but the same charCode values.