Update tiflash-configuration.md (#18828) (#18841)

tiflash/tiflash-configuration.md

@@ -207,6 +207,13 @@ delta_index_cache_size = 0
     ## see known issue [#5576](https://github.com/pingcap/tiflash/issues/5576).
     # dt_enable_logical_split = false
 
+    ## `max_threads` indicates the internal thread concurrency when TiFlash executes an MPP task.
+    ## The default value is 0. When it is set to 0,
+    ## TiFlash uses the number of CPU cores as the execution concurrency.
+    ## This parameter only takes effect
+    ## when the system variable `tidb_max_tiflash_threads` is set to -1.
+    max_threads = 0
+
     ## The memory usage limit for the generated intermediate data in a single query.
     ## When the value is an integer, the unit is byte. For example, 34359738368 means 32 GiB of memory limit, and 0 means no limit.
     ## When the value is a floating-point number in the range of [0.0, 1.0), it means the ratio of the allowed memory usage to the total memory of the node. For example, 0.8 means 80% of the total memory, and 0.0 means no limit.