[8.13] Add details on load balance behaviour with Logstash output (backport #1024)

[mergify]

This updates the two Logstash output settings pages (one for standalone agent and one for Fleet-manged agent) to provide more detail on the loadbalance setting.

To preview changes, see the loadbalance setting here.

Details are from the discussion in https://github.com/elastic/beats/issues/12567.

Closes: #1023

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This is an automatic backport of pull request #1024 done by [Mergify](https://mergify.com).

[github-actions]

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