Doc: Add relfrozenxid Tip to XID wraparound section.
VACUUM VERBOSE and autovacuum log reports were taught to report the
details of how VACUUM advanced relfrozenxid (and relminmxid) by commit
872770fd. Highlight this by adding a "Tip" to the documentation, next
to related discussion of age(relfrozenxid) monitoring.
Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-Wzk0C1O-MKkOrj4YAfsGRru2=cA2VQpqM-9R1HNuG3nFaQ@mail.gmail.com