Computational notebooks allow data scientists to express their ideas through a combination of code and documentation. However, data scientists often pay attention only to the code, and neglect creating or updating their documentation during quick iterations, which leads to challenges in sharing their notebooks with others and future selves. Inspired by human documentation practices from analyzing 80 highly-voted Kaggle notebooks, we design and implement Themisto, an automated documentation generation system to explore the Human-AI Collaboration opportunity in the code documentation scenario. Themisto facilitates the creation of different types of documentation via three approaches: a deep-learning-based approach to generate documentation for source code (fully automated), a query-based approach to retrieve the online API documentation for source code (fully automated), and a user prompt approach to motivate users to write more documentation (semi-automated). We evaluated Themisto in a within-subjects experiment with 24 data science practitioners, and found that automated documentation generation techniques reduced the time for writing documentation, reminded participants to document code they would have ignored, and improved participants satisfaction with their computational notebook.