Very carefully - and I mean extremely carefully - word it so that it describes what it does, while implying what it should do without confirming that it actually does that.
Also helps if you fix the bug or change the behavior, the docs are still technically correct. I'm only partially kidding, I swear I've seen this a million times in documentation I read.
How many engineers does it take to fix a bug?
Hardware Engineers: "None. We'll fix it in firmware."
Firmware Engineers: "None. We'll fix it in software."
Software Engineers: "None. We'll document it in the manual."
Technical Writers: "None. The user can figure it out." etc.
While I get the joke, as a technical writer, you might be surprised how often I've found myself as a defacto QA engineer:
Me: This is what you said it does, and this is what it actually seems to do. Which one is right?
Engineer: Shit.
Very carefully - and I mean extremely carefully - word it so that it describes what it does, while implying what it should do without confirming that it actually does that.
Also helps if you fix the bug or change the behavior, the docs are still technically correct. I'm only partially kidding, I swear I've seen this a million times in documentation I read.
The joke is awesome.
Sadly, monospaced fonts kill sarcasm.