Even as you type those words — '…never intended to be officially documented…' — you can't help but screw up your face.
That is complete and utter rubbish for two very good reasons.
First, you are officially documenting it right now. That is a thing that you are definitely doing. So if it's not supposed to be documented, then documenting that it's not supposed to be documented is the biggest paradox you've ever made. And while you've never worked on a temporal skip, that's still a reasonably big statement.
Secondly, you were told to document this object. Very specifically, this one. SCP-3939. The number was written multiple times in each message you received about it, from two different members of the O5 council, and not once was any correction made.
So it is supposed to be documented. Even if it's not, it's a moot point, because you're going to document it anyway, and if you get an XK in return then it's not really your fault. Your job, your Class 4 office, is more important.