The first google hit I get is a presentation that includes a very small config sample: http://www.hpts.ws/papers/2015/wilkes.pdf

The second hit is a google+ post wailing “arrrrgh, somebody reinvented borgcfg for Kubernetes” :)

I’m guessing the switch to YAML, even though it might be a bit tedious, was an explicit reaction to the fact that the ultra-programmable borgcfg turned out to cause some big headaches.