dashboard statement list: add a note to explain the blue bar and yell…

…ow line (#6189)

dashboard/dashboard-statement-list.md

@@ -22,6 +22,10 @@ You can use one of the following two methods to access the SQL statement summary
 
 All the data shown on the SQL statement summary page are from the TiDB statement summary tables. For more details about the tables, see [TiDB Statement Summary Tables](/statement-summary-tables.md).
 
+> **Note:**
+>
+> In the **Mean Latency** column of the SQL statement summary page, the blue bar indicates the average execution time. If there is a yellow line on the blue bar for an SQL statement, the left and right sides of the yellow line respectively represent the minimum and maximum execution time of the SQL statement during the recent data collection cycle. 
+
 ### Change Filters
 
 On the top of the SQL statement summary page, you can modify the time range of SQL executions to be displayed. You can also filter the list by database in which SQL statements are executed, or by SQL types. The following image shows all SQL executions over the recent data collection cycle (recent 30 minutes by default).