Clarify the details on how to set retention for LokiStorage

As per the logs we documented about enabling stream based retention but there is no clarity on how to set retention period to clear the logs from object storage.

We need to add the note in the doc to make customer aware that logs retention for object bucket should be configured on storage provider side

For cost-efficient logs pruning, it is recommended to set global retention on the object storage provider side. Pruning older logs is a cost free task provided by the storage provided & omits Loki's resource to be bound to the task of pruning logs.

Ref KCS https://access.redhat.com/solutions/7053212