[monitoring] Document all elasticsearch settings

[elasticmachine]

Original comment by @jbudz:

The docs at https://www.elastic.co/guide/en/kibana/5.6/monitoring-settings-kb.html only have elasticsearch.url. Can we document the other settings too?

It should be a pretty close match to the kibana settings at https://www.elastic.co/guide/en/kibana/current/settings.html

[elasticmachine]

Original comment by @inqueue:

This came up again recently with someone attempting to configure a monitoring cluster with TLS enabled. The doc says:

There are also xpack.monitoring.elasticsearch.* settings, which support the 
same values as Kibana configuration settings

This is very confusing to a newcomer and I myself had to look at Kibana source just to understand the structure of the configuration parameters for TLS/SSL. All settings supported in kibana.yml should be clearly documented, even if it means overlap with other product documentation.

[chrisronline]

One major thing we need to correct is the prefix change that will be in 7.7.

Other than that, here are the lists of configs (using the new prefix):

monitoring.enabled
monitoring.elasticsearch  (same as elasticsearch.* settings)
monitoring.ui.enabled
monitoring.ui.ccs.enabled
monitoring.ui.logs.index
monitoring.ui.max_bucket_size
monitoring.ui.elasticsearch  (same as elasticsearch.* settings except for the ones listed below)
monitoring.ui.elasticsearch.logFetchCount
monitoring.ui.container.elasticsearch.enabled
monitoring.ui.container.logstash.enabled
monitoring.ui.min_interval_seconds
monitoring.ui.show_license_expiration
monitoring.kibana.collection.enabled
monitoring.kibana.collection.interval
monitoring.cluster_alerts.enabled
monitoring.cluster_alerts.email_notifications.enabled
monitoring.cluster_alerts.email_notifications.email_address
monitoring.xpack_api_polling_frequency_millis
monitoring.agent.interval
monitoring.tests.cloud_detector.enabled

I don't think we use monitoring.agent.interval anymore, but the rest look legit. The monitoring.cluster_alerts will go away once we shift to using Kibana alerting (which is expected to land before 8.0)

[igoristic]

Adding a newly created option to the current list in: #18448 (comment)

monitoring.ui.container.apm.enabled: true

Also relates to:
#90157
#37644

[KOTungseth]

Closing due to inactivity. To further discuss, please reopen.