Deep neural networks (DNNs) routinely achieve state-of-the-art performance in a wide range of tasks. This case study reports on the development of onboarding (i.e., training) materials for a DNN-based medical AI Assistant to aid in the grading of prostate cancer. Specifically, we describe how the process of developing these materials deepened the team's understanding of end-user requirements, leading to changes in the development and assessment of the underlying machine learning model. In this sense, the onboarding materials served as a useful boundary object for a cross-functional team. We also present evidence of the utility of the subsequent onboarding materials by describing which information was found useful by participants in an experimental study.