Yep, this is exactly what happened. The OP didn't stop to consider the purpose of the exercise or what the interviewer was trying to get out of it. The details

Yep, this is exactly what happened. The OP didn't stop to consider the purpose of the exercise or what the interviewer was trying to get out of it. The details of the exercise aren't the point...and I'm sure the reason the interviewer likely gave off mixed signals about the extensive documentation is because time is limited and a good interviewer makes sure to keep the candidate on track so that there's enough time.