This is such an obvious idea that it may exist, in which case I'll delete.
I sometimes leave reviews of restaurants on TripAdvisor. Sometimes it's because I really, really hated the place, in which case I am happy to share that information with the rest of the world. More often, though, it's because
I really enjoyed it and want to recommend it.
But there's a problem. Sometimes, I've enjoyed a restaurant enough to want to recommend it, but there were one or two minor things that could have been better. Maybe a brilliant main dish was let down by one of the vegetables, or drinks orders were slow to arrive.
I don't really want to include these "buts" in my review, because on balance I want other people to visit so the place stays in business, so I tend not to put "buts" in a generally positive review.
What would be nice is if, on sites like TripAdvisor, I could add some extra comments that were only seen by the business owner. And yes, I know I could email the restaurant, but who does that?