Doc cautions about index repo with Redis Cluster

[kinsersh]

Due to https://github.com/spring-projects/spring-data-redis/issues/1111, applications can suffer from missing important Keyspace events as described at https://github.com/spring-projects/spring-session/issues/2230.

This adds a caution to the docs to help others avoid running into this problem and suggests an alternative.

Mitigates #2230.