Unfortunately, this (and any other use of the horn besides warning of an impending accident) CAN fall under the other use for horn I mentioned (trying to make someone mad enough to confront you).It's also for waking up someone (or interrupting his current nondriving activity) when he fails to drive after the light turns green.
Don't believe it?
Here are a couple of bumper stickers that make the point in a humorous fashion.
"Keep honking, I'm reloading."
"Honk if you've never seen an Uzi fired out of a car window."
Seriously, I have sometimes used the horn for the purpose you describe, but it's always with the understanding that I may be stirring up a hornet's nest. So far I've been lucky, but I've heard of serious incidents resulting from far less. You just never know what's going to push a person over the edge...
I actually had a person follow me for several miles after I pulled to the side of the road at night and flashed my high-beams at a piece of debris on the shoulder in an effort to determine what it was (a detached bumper). Evidently they were waiting to pull out onto the highway some distance ahead of me and interpreted my stop and the light flash as some sort of insult or affront. I didn't even notice them at the time--later after they followed me for some distance, I thought back over the incident and remembered that there had been a car waiting to turn onto the highway. The person followed me until I made my stop. He (with his family in the car) waited until I stopped the car and then pointed his car at mine and flashed his high-beams. Tit-for-tat, I suppose. I guess it just took him wrong...