Yeah now that I saw that other picture above where I was specifically looking for it I've started to notice it on pictures.I've seen or had what I'd describe more as a prominent forehead slope in most cases. Occasionally it's what you could describe as a slight bump if you wanted to. Or it can be a sort of notch, as in some angelfish. In some older males it may become more noticeable, still nothing like the bulging humps of some cichlids. If you ever saw that in a discus it would be an abnormal growth and you'd worry about the fish.
A vertical forehead profile is one thing, or a slight change in slope nearing the dorsal fin, but an overly notchy head would be considered undesirable by any of the discus people I've known. Sometimes an underfed or emaciated individual has a notchy looking head that goes away with better health and feeding.
It's easy enough to google "discus fish" and see loads of photos.