Doc: update VACUUM VERBOSE freezing tip.

Doc: update VACUUM VERBOSE freezing tip.

VACUUM VERBOSE/autovacuuming logging have reported on the number of
pages frozen by VACUUM since commit d977ffd9 added that capability.
This information is directly related to relfrozenxid advancement, so
update an older tip from the documentation about how relfrozenxid is
reported on by the same instrumentation code.  Now the tip directly
mentions newly frozen pages, too.