Document using a ConfigMap as a source of values for configuration expression resolution

As discussed on WFLY-9452, all management attributes in our configuration that support expressions support environment variables as a source of the value for the expression, using ${env.THE_VAR_NAME}. Further, it is a standard OpenShift capability to let users use a ConfigMap as a source of env vars:

https://kubernetes.io/docs/tasks/configure-pod-container/configure-pod-configmap/#define-container-environment-variables-using-configmap-data

The combination lets users put an expression in their standalone-openshift.xml and then drive the value of that expression from a ConfigMap. This seems like a good thinking to mention in the EAP on OS docs.

Note also the WFLY-11529 is a proposed enhancement to community docs to discuss in general how configuration expressions work. I don't know when that will be done though. It's about 8 years overdue!