There are two different questions you have asked here.
Any tool to document testing APIs?
You can use JMeter or Gatling to do API testing
Please provide some tools for documentation
-For documentations, you can use Swagger, if you are following OpenAPI standards
Swagger (Mainly used for: Document, Testing API, Versioning)
If you have your API customized, you can use
Output can be in html, pdf, docx
Hope that helps!