On more than one occasion in Back to the Future, Marty tries to warn 1955-Doc that he will be shot on the night he tests the DeLorean. The first time, he tells Marty to shut up and says something like "I don't want you to endanger my future the way you endangered your own future". But even from Marty's perspective in 1985, any potential consequences of telling the Doc about the shooting would only affect the "real" future, i.e., things that hadn't happened yet in any timeline.
This is very different from Marty's situation in 1955 - by changing the course of events, he was creating a very real possibility of negating his own existence (as well as the existence of his siblings). But 1955-Doc learning that Libyans would try to kill him in 30 years does not pose similar problems. No one who already exists will cease to exist, and the changes that would proceed from the Doc wearing body armor would have no effect on current events only things that had not yet come to pass (in any timeline). In fact, I can't see how it would change anything other than 1985-Doc dying.
Am I missing something here? How is Marty changing his own past anything like him changing the Doc's future? Is it actually dangerous for 1955-Doc to hear that he will be shot?