Missing required Elasticsearch permissions documentation

**Describe the feature:**
Currently I was not able to find permissions, that Janusgraph require to be able to successfully work with Elasticsearch. Would it be possible to add this section to documentation?

**Describe a specific use case for the feature:**
I think it would be beneficial for all users of Janusgraph to grant at least privilege to Elasticsearch user.

The issue does not name a documentation file, test, or entry point. Start by locating the existing JanusGraph documentation for Elasticsearch integration, then identify the permissions required for successful operation and document a least-privilege configuration; done means users can find the required permissions in the documentation.

Written by the indexing model from the issue text.