Undocumented as in "we included it in our 'CSS3-compliant selector engine' but do not mention anywhere that it is not actually part of the CSS3 spec, just something we thought was cool".
Now that we've gone all the way down the pedantry slide, have you anything of value to contribute, or was that it?
If you want to pick on something that claims compatibility, try Sizzle itself - http://sizzlejs.com/. In that case, you'll find a summary of alterations right at the top of its documentation https://github.com/jquery/sizzle/wiki .