While the docs are my first stop for info, baring that I look for examples in the SWI-Prolog source code at GitHub as also noted by Jan.
At GitHub to search the code you have to go into a repository, e.g. swip-devel, swish or one of the others.
Once in a repository then use search at the upper left of the page. I typically search just swip-devel.
Searching for round returns many hits
Not all of the hits are Prolog source code but it is real working code.
That is how I learned to use setup_call_cleanup