[enterprise-4.10] Remove incorrect info regarding UWM external labels #44324

GitHub issue:

https://github.com/openshift/openshift-docs/issues/44324

Which section(s) is the issue in?

https://docs.openshift.com/container-platform/4.9/monitoring/configuring-the-monitoring-stack.html#attaching-additional-labels-to-your-time-series-and-alerts_configuring-the-monitoring-stack

What needs fixing?

The section about attaching additional labels to the user-workload monitoring instance has an info box, that correctly states:
 In the openshift-user-workload-monitoring project, Prometheus handles metrics and Thanos Ruler handles alerting and recording rules. Setting externalLabels for prometheus in the user-workload-monitoring-config ConfigMap object will only configure external labels for metrics and not for any rules.
Yet the same section mentions attaching labels to alerts in the user-workload monitoring instance multiple times:
 To attach custom labels to all time series and alerts leaving the Prometheus instance that monitors user-defined projects:
We should be consistent here.