Clarify replica access for AS OF SYSTEM TIME

[jseldess]

Jesse Seldess commented:

Our docs on follower reads (also the topology page) explain how AS OF SYSTEM TIME leverages follower replicas to return reads faster. However, we don't explain how AS OF SYSTEM time works when there's no enterprise license or when there is a license but the interval is not far enough in the past to qualify for a follower read.

In both cases, the read accesses the leaseholder replica, not a follower replica. This adds network latency if the leaseholder is not the closest replica to the gateway node.

Jira Issue: DOC-452

[jseldess]

cc @kai-niemi, @ajwerner, @johnrk

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