IF a doctor considers you are suicidal, then you pose a risk to YOURSELF and I would imagine that gives them a duty to you - their patient - to do something about it.
It's a bit more complicated than that. If a patient is clearly barking, then they can be sectioned. However, if they say, 'Doctor, I'm 92. All my friends have died. My knees keep me awake at night. I'm not depressed, but I think I've had enough.' and they don't have signs of clinical depression then you would try to help them medically or practically, but almost certainly couldn't section them, and you couldn't break medical confidentiality to their relatives either.
The truth of the matter is that a lot of people are at some risk of killing themselves, but only a small proportion of these will benefit from being coerced onto a psychiatric ward.