modules/upi: Document that RHCOS is RHEL8
modules/upi: Document that RHCOS is RHEL8

So people know it has the same requirements/limits.

Co-Authored-By: Kathryn Alexander <50dd8c72987c196660bd3460fa2e803d80f71a7e@users.noreply.github.com>

vikram-redhat/openshift-docs,vikram-redhat/openshift-docs,vikram-redhat/openshift-docs,vikram-redhat/openshift-docs,vikram-redhat/openshift-docs