sync-diff-inspector: update required privilege for sync-diff-inspector (pingcap#21160) (pingcap#21173)

Author: ti-chi-bot

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

@@ -37,16 +37,15 @@ This guide introduces the key features of sync-diff-inspector and describes how
 
 ## Database privileges for sync-diff-inspector
 
-sync-diff-inspector needs to obtain the information of table schema and to query data. The required database privileges are as follows:
-
-* Upstream database
-    - `SELECT` (checks data for comparison)
-    - `SHOW_DATABASES` (views database name)
-    - `RELOAD` (views table schema)
-* Downstream database
-    - `SELECT` (checks data for comparison)
-    - `SHOW_DATABASES` (views database name)
-    - `RELOAD` (views table schema)
+To access table schemas and query data, sync-diff-inspector requires specific database privileges. Grant the following privileges on both the upstream and downstream databases:
+
+- `SELECT`: required to compare data.
+- `RELOAD`: required to view table schemas.
+
+> **Note**:
+> 
+> - **DO NOT** grant the [`SHOW DATABASES`](/sql-statements/sql-statement-show-databases.md) privilege on all databases (`*.*`). Otherwise, sync-diff-inspector will attempt to access inaccessible databases, which causes errors.
+> - For MySQL data sources, ensure that the [`skip_show_database`](https://dev.mysql.com/doc/refman/8.4/en/server-system-variables.html#sysvar_skip_show_database) system variable is set to `OFF`. If this variable is set to `ON`, the check might fail.
 
 ## Configuration file description