Document that log_autovacuum_min_duration can be used to monitor

autovacuum activity.

Per report from Marc Mamin

doc/src/sgml/maintenance.sgml

@@ -633,6 +633,8 @@ HINT:  Stop the postmaster and use a standalone backend to VACUUM in "mydb".
     the next database will be processed as soon as the first worker finishes.
     Each worker process will check each table within its database and
     execute <command>VACUUM</> and/or <command>ANALYZE</> as needed. 
+    <varname>log_autovacuum_min_duration</varname> can be used to monitor
+    autovacuum activity.
    </para>
 
    <para>