Doc: Clarify the role of aggressive VACUUMs.

Adjust the documentation's coverage of aggressive VACUUMs to make it
clearer that aggressive vacuuming isn't always strictly necessary.  It's
possible for non-aggressive VACUUMs to advance relfrozenxid/relminmxid
without fail, given the right workload conditions.

While this has always been true, it matters more with recent and pending
improvements to VACUUM.  These improvements make non-aggressive
vacuuming more likely to advance relfrozenxid/relminmxid in practice.
While this is an unrelated improvement to the docs, formally speaking,
it still doesn't seem worth backpatching.  So don't backpatch.

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