This is a question I ask myself a lot. I think that as the fear of dying itself, the impact on your relatives may be one of the strongest thing that keep you away from ctb. I know for sure it already dissuaded me in the past. In some way it is not fair... If we consider that we should live for ourself first, but I guess it is a natural feeling.
I decided to let my relatives know about what I will do when the time comes, so that they are not being left in uncertainty about my fate.
Regarding the content of the note itself, I thought about explaining a lot of things, but now I am thinking that most of my relatives know about my depression and my suicidal thoughts, so what is the point in explaining. They do not fully understand now (and I do not blame them for that), and it will not change when I'm gone... And somehow I think it must be hard enough to learn that a son/cousin/friend/coworker has passed away, so maybe leaving a long message with explanation of things they already know would be absurd.
A short message to explain that it was supposed to happen someday, and that I love them and thank them for loving me and being supportive all these years should be enough.