lightning: document logical-import-batch-{size,rows} (pingcap#16790)

tidb-lightning/tidb-lightning-configuration.md

@@ -238,6 +238,21 @@ strategy = ""
 # This parameter is introduced in v7.6.0. The default value is "16KiB". The value must be greater than or equal to `1B`. Note that if you only specify a number (for example, `16`), the unit is Byte instead of KiB.
 # block-size = "16KiB"
 
+# In Logical Import Mode, this parameter controls the size of each SQL statement executed on the downstream TiDB server.
+# This parameter is introduced in v8.0.0.
+# It specifies the expected size of the VALUES part of each INSERT or REPLACE statement in a single transaction.
+# This parameter is not a hard limit. The actual SQL executed might be longer or shorter, depending on the actual content imported.
+# The default value is "96KiB", which is optimized for import speed when TiDB Lightning is the only client of the cluster.
+# Due to the implementation details of TiDB Lightning, the value is capped at 96 KiB. Setting a larger value will not take effect.
+# You can decrease this value to reduce the stress on the cluster due to large transactions.
+# logical-import-batch-size = "96KiB"
+
+# In Logical Import Mode, this parameter controls the maximum number of rows inserted per transaction.
+# This parameter is introduced in v8.0.0. The default value is `65536` rows.
+# When both `logical-import-batch-size` and `logical-import-batch-rows` are specified, the parameter whose value reaches its threshold first will take effect.
+# You can decrease this value to reduce the stress on the cluster due to large transactions.
+# logical-import-batch-rows = 65536
+
 [mydumper]
 # Block size for file reading. Keep it longer than the longest string of the data source.
 read-block-size = "64KiB" # default value