Remove outdated recommendation for manual VACUUM

Remove outdated recommendation for manual VACUUM

We have had a working and tunable autovacuum
for at least a decade now, so remove the recommendation to
manually vacuum tables at least every night.
Autovacuum is now also triggered by INSERTs, so we can also
remove the recommendation to run VACUUM (ANALYZE) after lots
of INSERTs or DELETEs.

Instead, suggest using autovacuum by moving the respective
paragraph up to where the importance of VACUUM is emphasized.

Author: Laurenz Albe <laurenz.albe@cybertec.at>
Reviewed-By: Magnus Hagander, Peter Geoghegan
Discussion: https://postgr.es/m/6f5e3da98fec14640f389d7b84c3b413833697f4.camel@cybertec.at