[Website]: Specify the target cluster_uuid (optional) step

### Before you submit

- [x] This issue is about a documentation page, flow, or piece of content.

### Type of issue

Inaccurate

### What documentation page or section is affected

https://www.elastic.co/docs/reference/logstash/monitoring-with-elastic-agent

### What happened?

We should clarify when using the `monitoring.cluster_uuid` is optional.

If Logstash writes to more than 1 ES cluster, then Logstash cannot "guess" what is the Cluster UUID and it's mandatory to explicitly set the `monitoring.cluster_uuid`.

### Additional info

To be reviewed by @consulthys / @robbavey