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

Author: michaelpq

doc/src/sgml/monitoring.sgml

@@ -6147,8 +6147,9 @@ SELECT pg_stat_get_backend_pid(s.backendid) AS pid,
        <command>VACUUM</command> is currently vacuuming the indexes.  If a table has
        any indexes, this will happen at least once per vacuum, after the heap
        has been completely scanned.  It may happen multiple times per vacuum
-       if <xref linkend="guc-maintenance-work-mem"/> is insufficient to
-       store the number of dead tuples found.
+       if <xref linkend="guc-maintenance-work-mem"/> (or, in the case of autovacuum,
+       <xref linkend="guc-autovacuum-work-mem"/> if set) is insufficient to store
+       the number of dead tuples found.
      </entry>
     </row>
     <row>