[Stack monitoring] Doc improvements

[mashhurs]

Description

When we monitor LS and do not monitor ES with either metricbeat or self-monitoring, we don't see the LS cluster dashboard unless we setup the ES monitoring. It is because cluster UUID is decided based on ES monitoring (.monitoring-es*) data.

• Self-monitoring
  • Caveat (mostly confusing) is, we enabled self-monitoring in logstash.yml but we don't see it unless we enable self monitoring on Kibana.
• Metricbeat monitoring
  • Similarly with self-monitoring, the caveat, we configured cluster_uuid in logstash.yml but we don't see it unless we enable elasticsearch-xpack module of the metricbeat and configure to monitor the Elasticsearch as well.

Some confusing examples users asked:

• Here is the one of the discussions I have seen: https://discuss.elastic.co/t/logstash-monitoring-data-not-showing-in-stack-monitoring/256920

Expectation

We need to highlight these cases in the docs to clear out the confusions.