fix and document aggregate cluster behavior

The aggregate cluster documentation describes how load balancing works with an aggregate cluster, but it doesn't describe how any of the other cluster behavior works.

Here are a few off-the-cuff examples (not a comprehensive list):

• What happens for circuit breakers? Does the circuit breaking config in the aggregate cluster get used, or does it somehow delegate to the circuit breaking configs in the clusters being delegated to?
• How does load reporting work? Does the load get reported as belonging to the underlying clusters or the aggregate cluster?
• How does outlier detection work?

@yxue, can you please update the documentation to explain how these sorts of things are supposed to work? Ideally, the doc should cover the behavior for every feature configurable via the Cluster xDS resource.

Thanks!