Document ES public instance and limits

[andreibancioiu]

Description of the pull request (what is new / what has changed)

Document Elasticsearch public instance, plus its limits.

Did you test the changes locally ?

• [x] yes
• [ ] no

Which category (categories) does this pull request belong to?

• [ ] document new feature
• [ ] update documentation that is not relevant anymore
• [x] add examples or more information about a component
• [ ] fix grammar issues
• [ ] other