Suggestion: Add this CSS to [[MediaWiki:Common.css]]

That was my question too :P

Anyways, the "problem" here is two-fold: in one hand, it is important to show that there is no documentation message, so that you can invite the user to post a documentation of it if he/she knows it; on the other hand it is nice to show that there actually is a documentation message vs. there is none (which currently isn't very clear because of the aforementioned reason). The best solution in my mind is to separate the two visually, and the way I did it on Meta is one of the simplest: show an icon of there is documentation, don't show one if there isn't. I think this has an advantage over, e.g., colour coding, since it makes it obvious when there is an information message.

How does having an icon have advantage over color? The way I see it, if the entire message "There is no documentation..." was on a red background, that'd be much more noticeable than an icon.