Clarify replica access for AS OF SYSTEM TIME

Jesse Seldess commented:

Our docs on [follower reads](https://www.cockroachlabs.com/docs/v19.2/follower-reads.html) (also the [topology page](https://www.cockroachlabs.com/docs/v19.2/topology-follower-reads.html)) explain how AS OF SYSTEM TIME leverages **follower** replicas to return reads faster. However, we don't explain how [AS OF SYSTEM](https://www.cockroachlabs.com/docs/v19.2/as-of-system-time.html) 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