There's a difference between advertising and answering someone's question.

If you were to run around a major city spamming the URL without provocation, then sure. I can see how that's advertising.

If you are responding to a whisper in a conversational manner, I see nothing wrong with recommending an online resource for clarification.

If you are super concerned, why not link them a post directly on the official World of Warcraft forum? Like Belfaire's post with the URL of http://forums.worldofwarcraft.com/th...88579356&sid=1 -- that way you'll be using an official world of warcraft source.

In fact, if you want further clarification on whether or not your use of external websites during conversations, why not post on the Customer Service forums on the official WoW forums? The GMs there are pretty good at reponding to questions -- and I find their responses are a bit more thorough (and you'll also have a lasting record that isn't just screenshots) compared to in-game ticketed GMs.