api-ref: update node.resource_class description
api-ref: update node.resource_class description

The future of scheduling with custom resource classes is now!
Actually it has been for a couple of years. This drops the
"future" part of the parameter description and links to the
docs on configuring nova flavors with custom resource classes
for scheduling with the placement service.

Change-Id: Id904acdf305096738604eacb225136f1f9c03338

openstack/ironic,openstack/ironic