I recently had the same problem and I did not find anything. So I created a simple tool to do it. It is a very basic javascript application written with react.js. At the moment it is very basic, but I’m working on it, so any feature request is welcome.
You can use it for free at: http://www.vernizzis.it/docman/
Plus you can get the code and modify it at: https://github.com/davidevernizzi/docman
Any comment is more than welcome.
UPDATE1: looking around I also found this project: https://github.com/JakeWorrell/docodile
and this feature request: https://github.com/a85/POSTMan-Chrome-Extension/issues/204
UPDATE2: the feature request (see UPDATE1) has been closed and the possibility of creating documentation has been added to Postman cloud. Here is the feature request closing comment:
This is available as part of Postman Cloud: http://blog.getpostman.com/2015/12/10/introducing-postman-cloud/. Cloud hosts, updates and maintains documentation for everyone. I’d suggest http://docman.launchrock.com/ and https://github.com/JakeWorrell/docodile for generating one-time documentation from Collections.