Update fine grained shuffle stream count description (#17114)

system-variables.md

@@ -6025,7 +6025,7 @@ For details, see [Identify Slow Queries](/identify-slow-queries.md).
 - When the window function is pushed down to TiFlash for execution, you can use this variable to control the concurrency level of the window function execution. The possible values are as follows:
 
     * -1: the Fine Grained Shuffle feature is disabled. The window function pushed down to TiFlash is executed in a single thread.
-    * 0: the Fine Grained Shuffle feature is enabled. If [`tidb_max_tiflash_threads`](/system-variables.md#tidb_max_tiflash_threads-new-in-v610) is set to a valid value (greater than 0), then `tiflash_fine_grained_shuffle_stream_count` is set to the value of [`tidb_max_tiflash_threads`](/system-variables.md#tidb_max_tiflash_threads-new-in-v610). Otherwise, it is set to 8. The actual concurrency level of the window function on TiFlash is: min(`tiflash_fine_grained_shuffle_stream_count`, the number of physical threads on TiFlash nodes).
+    * 0: the Fine Grained Shuffle feature is enabled. If [`tidb_max_tiflash_threads`](/system-variables.md#tidb_max_tiflash_threads-new-in-v610) is set to a valid value (greater than 0), then `tiflash_fine_grained_shuffle_stream_count` is set to the value of [`tidb_max_tiflash_threads`](/system-variables.md#tidb_max_tiflash_threads-new-in-v610). Otherwise, it is automatically estimated based on the CPU resources of the TiFlash compute node. The actual concurrency level of the window function on TiFlash is: min(`tiflash_fine_grained_shuffle_stream_count`, the number of physical threads on TiFlash nodes).
     * Integer greater than 0: the Fine Grained Shuffle feature is enabled. The window function pushed down to TiFlash is executed in multiple threads. The concurrency level is: min(`tiflash_fine_grained_shuffle_stream_count`, the number of physical threads on TiFlash nodes).
 - Theoretically, the performance of the window function increases linearly with this value. However, if the value exceeds the actual number of physical threads, it instead leads to performance degradation.