Effective API documentation is crucial for the usability of an API, as it acts as the bridge between the API itself and the developers who utilize it. Comprehensive documentation should include essential elements such as an overview, authentication details, and error handling, along with code snippets to demonstrate real-world application. The design should facilitate easy navigation through dynamic pages, using elements like anchor text and a two-pane view for simultaneous access to explanations and code. To enhance usability, features like support buttons and options for readers to suggest edits can improve the documentation's quality and responsiveness. Additionally, providing resources such as quick start guides, tutorials, and API explorers can aid in developer education and onboarding. Tools like analytics and support software are also recommended to monitor API usage and manage developer inquiries effectively. The checklist aims to cater to both solo developers and large teams, suggesting an incremental approach to building documentation, starting with the essentials and expanding over time.