pgsql: doc: Improve description of index vacuuming with GUCs

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

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/52f8575a9e75e67ed7a7ae1585be28a85e85ae0e

Modified Files
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doc/src/sgml/monitoring.sgml | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)