Include example models of payloads in Swagger documentation

Swagger has been introduced in JBPM-4480 and it really helps communicate with KIE Server REST services. However, the last missing bit is a model of a body/payload which has to be sent in the particular POST/PUT request. This is currently missing in the documentation so users have to either know how the body looks like or find it in the documentation/on the Internet. Currently, when you open an endpoint there is only this body snippet:

<?xml version="1.0" encoding="UTF-8"?>
<!-- XML example cannot be generated -->

So I think it should be possible to add examples.

I will assign Duncan to this, but feel free to reassign as you wish.