modules/update-service-overview: "both OpenShift and RHCOS" -> "OpenShift, including RHCOS"
modules/update-service-overview: "both OpenShift and RHCOS" -> "OpenShift, including RHCOS"

The RHCOS mention is from way back in 1ac3751e97 (some updates per
Clayton, 2018-11-26, #12880).  But:

  $ git --no-pager grep -h supported modules/rhcos-about.adoc
  {op-system} is supported only as a component of {product-title} {product-version} for all {product-title} machines....

This commit rephrases the update docs to put RHCOS under OpenShift, so
folks don't get ideas and think it is a stand-alone product.

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