Document enhancements in DocumentNodeStore in 1.6

This task is meant to collect and refer work done in 1.6 release which needs to be documented in Oak docs. Specially those enhancements which impact system administration or new features which need be to enabled as per requirements should be documented

Issues in documentmk, mongomk, rdbmk jql

• OAK-1312 - Bundle nodes into a document
• OAK-4180 - Use another NodeStore as a local cache for a remote Document store