stale read: update the description of tidb_external_ts (#20667)

Author: Oreoxmt

Diff for: stale-read.md

@@ -35,7 +35,7 @@ TiDB provides the methods of performing Stale Read at the statement level, the s
 - Session level
     - Specifying a time range: In a session, if you need TiDB to read the data as new as possible within a time range in subsequent queries without violating the isolation level, you can specify the time range by setting the `tidb_read_staleness` system variable. For detailed usage, refer to [`tidb_read_staleness`](/tidb-read-staleness.md).
 
-Besides, TiDB provides a way to specify an exact point in time by setting the [`tidb_external_ts`](/system-variables.md#tidb_external_ts-new-in-v640) system variable on session or global level. For detailed usage, refer to [Perform Stale Read Using `tidb_external_ts`](/tidb-external-ts.md).
+Besides, TiDB provides a way to specify an exact point in time at the session or global level by setting the [`tidb_external_ts`](/system-variables.md#tidb_external_ts-new-in-v640) and [`tidb_enable_external_ts_read`](/system-variables.md#tidb_enable_external_ts_read-new-in-v640) system variables. For detailed usage, refer to [Perform Stale Read Using `tidb_external_ts`](/tidb-external-ts.md).
 
 ### Reduce Stale Read latency