Expand comment on how pg_upgrade is turning off autovacuum.

contrib/pg_upgrade/server.c

@@ -172,7 +172,9 @@ start_postmaster(ClusterInfo *cluster, bool quiet)
 	 *
 	 * Using autovacuum=off disables cleanup vacuum and analyze, but
 	 * freeze vacuums can still happen, so we set
-	 * autovacuum_freeze_max_age high too.
+	 * autovacuum_freeze_max_age very high.  We assume all datfrozenxid and
+	 * relfrozen values are less than a gap of 2000000000 from the current
+	 * xid counter, so autovacuum will not touch them.
 	 */	
 	snprintf(cmd, sizeof(cmd),
 			 SYSTEMQUOTE "\"%s/pg_ctl\" -l \"%s\" -D \"%s\" "