Documented impact of using Redis caching instead of ehcaching

@Calle_Hedberg Redis is the recommended cache provider for large instances , especially for the ones which are running in a cluster (multiple nodes). However, it is not a replacement for ehcache. Ehcache still serves as the applications hibernate second level cache. But redis is used for other caching use cases and as a central user session repository (useful when dhis2 is running in a cluster).