cherry pick #3661 to release-3.1 (#3667)

Signed-off-by: ti-srebot <ti-srebot@pingcap.com>

Co-authored-by: JoyinQ <56883733+Joyinqin@users.noreply.github.com>

sync-diff-inspector/sync-diff-inspector-overview.md

@@ -22,7 +22,7 @@ This guide introduces the key features of sync-diff-inspector and describes how
 
 ### Restrictions
 
-* At present, online check is not supported. Ensure that no data is written into the upstream-downstream checklist, and that data in a certain range is not changed. You can check data in this range by setting `range`.
+* Online check is not supported for data migration between MySQL and TiDB. Ensure that no data is written into the upstream-downstream checklist, and that data in a certain range is not changed. You can check data in this range by setting `range`.
 
 * `JSON`, `BIT`, `BINARY`, `BLOB` and other types of data are not supported. When you perform a data check, you need to set `ignore-columns` to skip checking these types of data.
 

tidb-binlog/tidb-binlog-faq.md

@@ -14,6 +14,10 @@ This document collects the frequently asked questions (FAQs) about TiDB Binlog.
 
 - There is a slight performance impact on `INSERT`, `DELETE` and `UPDATE` transactions. In latency, a p-binlog is written concurrently in the TiKV prewrite stage before the transactions are committed. Generally, writing binlog is faster than TiKV prewrite, so it does not increase latency. You can check the response time of writing binlog in Pump's monitoring panel.
 
+## How high is the replication latency of TiDB Binlog?
+
+The latency of TiDB Binlog replication is measured in seconds, which is generally about 3 seconds during off-peak hours.
+
 ## What privileges does Drainer need to replicate data to the downstream MySQL or TiDB cluster?
 
 To replicate data to the downstream MySQL or TiDB cluster, Drainer must have the following privileges: