Node.js or Express.js REST API document generator

Swagger is an amazing project for auto generating API documentation. It includes an Express module.

http://swagger.io/

https://github.com/wordnik/swagger-node-express

What you are looking for is a good JavaScript documentation generator. I found a decent one here http://apidocjs.com/example/. This will allow you to use just Express as OP asked.

Swagger isn't for generating docs, it's for making APIs. So you are going to need to learn a full system to get that feature.

I found a couple more here that I haven't fully looked into yet which seem promising:
Docco here JSDoc here and an article on Documentor for Node

I found this library to be very useful when it comes to keeping your documentation and route declaration logic close together:

https://www.npmjs.com/package/swagger-jsdoc

If you use Postman while developing you might already have a Postman collection containing the most relevant calls. If that is the case Postman has a docs feature https://learning.postman.com/docs/publishing-your-api/documenting-your-api/