[Kibana Monitoring settings] monitoring.ui.elasticsearch.ssl.certificateAuthorities not documented in 7.9.1

[Erni]

Kibana version: 7.9.1

Elasticsearch version: 7.9.1

Describe the feature:
In 6.x we had these setting to configure monitoring. For example:

xpack.monitoring.elasticsearch.url: "https://xxx.yy.a.bbb:9200"
xpack.monitoring.elasticsearch.username: monitoring
xpack.monitoring.elasticsearch.password: XXXXXX
xpack.monitoring.elasticsearch.ssl.certificateAuthorities: [ "/etc/kibana/certificates/ca_mon.crt" ]

Now, in 7.9 we have these ones:

monitoring.ui.elasticsearch.hosts: ["https://xxx.yy.a.bbb:9200"]
monitoring.ui.elasticsearch.username: monitoring
monitoring.ui.elasticsearch.password: XXXXXX
monitoring.ui.elasticsearch.ssl.certificateAuthorities: [ "/etc/kibana/certificates/ca_mon.crt" ]

The thing is that the 7.9.1 Docs do not mention anything about this last setting monitoring.ui.elasticsearch.ssl.certificateAuthorities. While in the 6.8 Docs it was properly specified.
So the needed change is to add in the 7.9.1 Docs this setting so users can know about it.

[elasticmachine]

Pinging @elastic/stack-monitoring (Team:Monitoring)

[elasticmachine]

Pinging @elastic/kibana-docs (Team:Docs)