Mark vacuum_defer_cleanup_age as PGC_POSTMASTER.

Following bug analysis of #7819 by Tom Lane

doc/src/sgml/config.sgml

@@ -2356,8 +2356,7 @@ include 'filename'
         is measured in terms of number of write transactions occurring on the
         primary server, it is difficult to predict just how much additional
         grace time will be made available to standby queries.
-        This parameter can only be set in the <filename>postgresql.conf</>
-        file or on the server command line.
+        This parameter can only be set at server start.
        </para>
        <para>
         You should also consider setting <varname>hot_standby_feedback</>

src/backend/utils/misc/guc.c

@@ -1881,7 +1881,12 @@ static struct config_int ConfigureNamesInt[] =
 	},
 
 	{
-		{"vacuum_defer_cleanup_age", PGC_SIGHUP, REPLICATION_MASTER,
+		/*
+		 * Setting this to a higher value without restarting postmaster
+		 * can cause various bugs in TOAST object removal, CLUSTER and
+		 * possibly other places, so this must be changed only on restart.
+		 */
+		{"vacuum_defer_cleanup_age", PGC_POSTMASTER, REPLICATION_MASTER,
 			gettext_noop("Number of transactions by which VACUUM and HOT cleanup should be deferred, if any."),
 			NULL
 		},