Document using clusterAllReplicas and merge to correctly query system tables

[cwurm]

Because system tables are local per node and can be versioned (query_log.1, .2, etc.) the correct syntax to query all data esp. in ClickHouse Cloud is something like FROM clusterAllReplicas('default', merge('system', '^query_log.*')). We should document this somewhere, probably https://clickhouse.com/docs/operations/system-tables/overview#system-tables-in-clickhouse-cloud.

[gingerwizard]

We do @cwurm on each table at the top.

We'll add a note to the section https://clickhouse.com/docs/operations/system-tables/overview#system-tables-in-clickhouse-cloud as well

[gingerwizard]

ClickHouse/ClickHouse#79090