Document when to use GlobalOpenTelemetry in Java

[svrnm]

We might need to do another round on thinking about how to incorporate GlobaOpenTelemetry best (maybe it requires it's own section somewhere in the "Setup" section?)

I think even a small section that focuses just on that would be a good idea 👍

Originally posted by @mateuszrzeszutek in #3144 (comment)

[svrnm]

@open-telemetry/java-approvers is this covered by the updated docs?

[jack-berg]

Yes this is covered here: https://opentelemetry.io/docs/languages/java/api/#globalopentelemetry