The title says everything. You are probably not going to be the one using the REST API, others will. Obviously, even if you design a very intuitive set of endpoints, developers will still need to know the whole set of available endpoints, what each of them does, what optional parameters are available, and so on.

Write as much documentation as possible, and keep it up to date. Take a look at other documented APIs to gather ideas on how to display the information. There are plenty of templates and tools that will help you deliver a well-presented documentation, but you are the one that has to be consistent and methodical. Developers have a special hate towards documenting anything, but we also like to find clear and beautifully presented documentation when we need to use someone else's APIs.