That's very contextually sensitive and largely up to the discretion of the moderator. I would say that it only really applies if a user is doing it implicitly to "attack" another user, especially if the information that the person is digging up isn't funny and just steers them towards a completely regular person, but the only situation I know of where this happened is when someone tried to dox Hollywood and wound up digging out a completely normal, unrelated person.
Basically there's not really a defined rule for it other than don't be a dick and try to dox users just because they made you ass-blasted, and if you absolutely have to, only do it if the user has lots of funny skeletons in their closet, with bonus points awarded for irony or hypocrisy. IE: If someone on here spent all of their time in the Zoosadism threads lambasting the POIs in the thread, and then it turned out they're a degenerate dog-fucker, that's probably going to pique the forum's interest.