My note would at least explain that it was not their fault.
I would also leave information about how the burial is to be handled, as well as some other documents.
Should my family want more answers? My journals go into heavy detail about most of my thoughts, dating from my childhood to adulthood, and they continue to this day.
That said, knowing how my family is, I know regardless of how I word my note or the depth of all my journals, my family will not care about what life trauma I have, what I have experienced, what was done to me etc..
They will simply hand my body over to whom I have listed, and they will not miss or grieve my cashing-out.