Add some documentation about AWS Elasticsearch Service

In particular we should have a note which indicates that Nuxeo will not work with AWS Elasticsearch configured with IAM role/user because we do not implement signed request

https://docs.aws.amazon.com/elasticsearch-service/latest/developerguide/es-request-signing.html#es-request-signing-java

Ideally the new information would go to the Elasticsearch Setup page: https://doc.nuxeo.com/nxdoc/elasticsearch-setup/