Raising an issue about missing documentation seems like the righ way to go!
I am quite sure that UCP (management plane for Docker Enterprise) lists all labels for a node. Though, i won’t have access to UCP before my vacation is over.
Placement constraints do work. I use them at work regulary. Though, mine are usualy node.role==master or node.labels.{custom label}=={value for the custom label}.
The custom labels are created using “docker node update --label-add mylabel=myvalue {node name}”