The number one thing killing me about auto-generated API documentation is when an API takes a "string" that is really an enum and the generated documentation doesn't list out the enum values. Your only recourse is to read the code if it's open source and available in some obvious place, or hope there's a GUI alternative to the API with a pick list, select every option on the list, and intercept the internal API call it makes to get the string representation of each allowed value.
Looking at you, Nexus Repository Manager.