docs(clustered): add warning for system tables

[jdockerty]

When accessing system tables, it should be noted that this is currently not stable and can affect performance of production systems.

influxctl provides this warning through a prompt, but if a customer accesses their catalog directly they will be unaware. We should include a mention of this in the documentation so that we have extra coverage.

• [ ] Signed the InfluxData CLA (if necessary)
• [ ] Rebased/mergeable