Add note about extended key usage for Logstash output

[kilfoyle]

This adds the note in green to the Configure SSL/TLS for the Logstash output page.

In the docs issue @jguay asked: "Potentially check we use same libbeat code for logstash output managed by fleet so requirements documented for beats would apply equally"

@kpollich Would you be able to help me with Julien's question, i.e, to verify if this requirement applies to the Fleet Logstash output as it does to Filebeat?

Assuming that I should merge this PR, I'll then open another one for the 9.0+ docs.

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