doc: Improve description of index vacuuming with GUCs
Index vacuums may happen multiple times depending on the number of dead
tuples stored, as of maintenance_work_mem for a manual VACUUM. For
autovacuum, this is controlled by autovacuum_work_mem instead, if set.
The documentation mentioned the former, but not the latter in the
context of autovacuum.
Reported-by: Nikolai Berkoff
Author: Laurenz Albe, Euler Taveira
Discussion: https://postgr.es/m/
161545365522.10134.
12195402324485546870@wrigleys.postgresql.org
Backpatch-through: 9.6