update document to use tiflash in tispark (pingcap#4382)

* update document to use tiflash in tispark

* Update tiflash/use-tiflash.md

* Update use-tiflash.md

Co-authored-by: TomShawn <41534398+TomShawn@users.noreply.github.com>

tiflash/use-tiflash.md

@@ -193,25 +193,25 @@ In the above three ways of reading TiFlash replicas, engine isolation specifies
 
 ## Use TiSpark to read TiFlash replicas
 
-Currently, you can use TiSpark to read TiFlash replicas in a method similar to the engine isolation in TiDB. This method is to configure the `spark.tispark.use.tiflash` parameter to `true` (or `false`).
+Currently, you can use TiSpark to read TiFlash replicas in a method similar to the engine isolation in TiDB. This method is to configure the `spark.tispark.isolation_read_engines` parameter. The parameter value defaults to `tikv,tiflash`, which means that TiDB reads data from TiFlash or from TiKV according to CBO's selection. If you set the parameter value to `tiflash`, it means that TiDB forcibly reads data from TiFlash.
 
 > **Notes**
 >
 > When this parameter is set to `true`, only the TiFlash replicas of all tables involved in the query are read and these tables must have TiFlash replicas; for tables that do not have TiFlash replicas, an error is reported. When this parameter is set to `false`, only the TiKV replica is read.
 
-You can configure this parameter in either of the following ways:
+You can configure this parameter in one of the following ways:
 
 * Add the following item in the `spark-defaults.conf` file:
 
     ```
-    spark.tispark.use.tiflash true
+    spark.tispark.isolation_read_engines tiflash
     ```
 
-* Add `--conf spark.tispark.use.tiflash=true` in the initialization command when initializing Spark shell or Thrift server.
+* Add `--conf spark.tispark.isolation_read_engines=tiflash` in the initialization command when initializing Spark shell or Thrift server.
 
-* Set `spark.conf.set("spark.tispark.use.tiflash", true)` in Spark shell in a real-time manner.
+* Set `spark.conf.set("spark.tispark.isolation_read_engines", "tiflash")` in Spark shell in a real-time manner.
 
-* Set `set spark.tispark.use.tiflash=true` in Thrift server after the server is connected via beeline.
+* Set `set spark.tispark.isolation_read_engines=tiflash` in Thrift server after the server is connected via beeline.
 
 ## Supported push-down calculations