Related OpenShift documentation is Updating a cluster within a minor version by using the CLI.
Available upgrades are managed by https://github.com/openshift/cincinnati-graph-data/tree/master/channels.
Individual graph edges can be blocked by https://github.com/openshift/cincinnati-graph-data/tree/master/blocked-edges.
KCS that is dynamically built and communicates why edges are blocked: https://access.redhat.com/node/6103381