I find it's pretty easy to put a link in a comment to the [on-topic help page](http://serverfault.com/help/on-topic) and possibly a related meta post (or canonical question, such as for the licensing questions).

A great deal of these off topic questions are covered by the bold text near the top of that page.  If someone can't even be bothered to read that far (or click on the help link in the first place), I'm not sure another help page that is more concise will help them.