Include indexes on xattrs in Couchbase Server documentation

In the server side documentation, I am unable to find any document that explains global secondary indexes created on xattrs.

Sync gateway is an internal user of indexes on xattrs.

I found one document that explains basics of n1ql/gsi with respect to sync gateway, but more details (like which indexes are created, and how users can consume information related to these indexes) can be added to this page https://docs.couchbase.com/sync-gateway/current/indexing.html . DOC-9398 is already reported for updating the sync gateway documents.

This ticket requests following updates to Couchbase Server side documentation.

1. Add a document to explain global secondary indexes created on xattrs.
2. How to consume the information about these indexes.
3. What is the impact of using "retain_deleted_xattr" while creating an index - and how to use this.

A possible place for this information can be a link within this page: https://docs.couchbase.com/server/current/learn/services-and-indexes/indexes/global-secondary-indexes.html

More details about requirements for such a document can be found in the linked CBSE.