Hi Patrick,
Oddly enough, I cannot seem to be able to just "add" instructions to a quiz that already exists. If I try to edit the quiz this way, the description doesn't change for some reason. I found that I can do this only if I am creating the quiz in the same step. Using the following logic should work if you do not have the quizzes uploaded already:
POST /api/v1/courses/:course_id/quizzes/
BODY
quiz[title] = "Quiz A"
quiz[description] = "These are the instructions for the quiz"
You can write a loop that creates multiple quizzes and just inserts these instructions as they are created. Sorry if this does not resolve the issue entirely.