Fix autovacuum_vacuum_max_threshold's GUC description.

Most GUCs that accept a special value to disable the feature
mention it in their GUC description.  This commit adds that
information to autovacuum_vacuum_max_threshold's description.

Oversight in commit 306dc520b9.

src/backend/utils/misc/guc_tables.c

@@ -3428,7 +3428,7 @@ struct config_int ConfigureNamesInt[] =
 	},
 	{
 		{"autovacuum_vacuum_max_threshold", PGC_SIGHUP, VACUUM_AUTOVACUUM,
-			gettext_noop("Maximum number of tuple updates or deletes prior to vacuum."),
+			gettext_noop("Maximum number of tuple updates or deletes prior to vacuum, or -1 to disable maximum threshold."),
 			NULL
 		},
 		&autovacuum_vac_max_thresh,