Yeah, they’re not really documented anywhere (except in WebClientFactoryImpl.java) 
And I only found them there while trying to troubleshoot a problem someone was having with HttpUtil.downloadImage.
I was thinking that WebClientFactoryImpl could set the values based on the number of cores in the box. That way it would be memory friendly for Pi’s, yet not fail in mysterious ways for more capable servers.
OTOH, there could be a doc that suggests some runtime.cfg parameters for more capable servers. It would include these parameters, as well as threads for rules, etc. Maybe also java memory settings…