system variables: correct the description of `tidb_allow_fallback_to_…

…tikv` (#6868) (#6943)

system-variables.md

@@ -383,7 +383,7 @@ mysql> SELECT * FROM t1;
 
 - Scope: SESSION | GLOBAL
 - Default value: ""
-- This variable is used to specify a list of storage engines that might fall back to TiKV. If the execution of a SQL statement fails due to a failure of the specified storage engine in the list, TiDB retries executing this SQL statement with TiKV. This variable can be set to "" or "tiflash". When this variable is set to "tiflash", if the execution of a SQL statement fails due to a failure of TiFlash, TiDB retries executing this SQL statement with TiKV.
+- This variable is used to specify a list of storage engines that might fall back to TiKV. If the execution of a SQL statement fails due to a failure of the specified storage engine in the list, TiDB retries executing this SQL statement with TiKV. This variable can be set to "" or "tiflash". When this variable is set to "tiflash", if TiFlash returns a timeout error (error code: ErrTiFlashServerTimeout), TiDB retries executing this SQL statement with TiKV.
 
 ### tidb_allow_function_for_expression_index <span class="version-mark">New in v5.2.0</span>