changefeedccl: document high-water-mark semantics in REGIONAL BY ROW tables during network partition

A client uses change-feeds to keep an up-to-date client-side cache of a table. This client requires observing events in a global order, so they wait for high-water-marks to re-order previously-received events client-side before consuming them. When this client applies this approach to a `REGIONAL BY ROW` table, what is the expected semantic of high-water-marks delivered in the change-feed during a network partition between regions that the table has rows for?

If the client wants to keep at the very least a strongly consistent view of local data in the table, would it suffice to issue a geo-filtered change-feed, and would the high-water marks for those filtered change-feeds be delivered when the change-feed in question has seen all relevant events up to the mark, or are the high-water-marks still table-global?

Jira issue: CRDB-22219

Epic CRDB-21737