[docs](broker load) add doc for property load_parallelism (#13041)

docs/en/docs/sql-manual/sql-reference/Data-Manipulation-Statements/Load/BROKER-LOAD.md

@@ -168,6 +168,11 @@ WITH BROKER broker_name
 
     Specify the time zone for some functions that are affected by time zones, such as `strftime/alignment_timestamp/from_unixtime`, etc. Please refer to the [timezone](../../../../advanced/time-zone) documentation for details. If not specified, the "Asia/Shanghai" timezone is used
 
+  - `load_parallelism`
+
+    It allows the user to set the parallelism of the load execution plan
+    on a single node when the broker load is submitted, default value is 1.
+
 ### Example
 
 1. Import a batch of data from HDFS

docs/zh-CN/docs/sql-manual/sql-reference/Data-Manipulation-Statements/Load/BROKER-LOAD.md

@@ -168,6 +168,10 @@ WITH BROKER broker_name
 
     指定某些受时区影响的函数的时区，如 `strftime/alignment_timestamp/from_unixtime` 等等，具体请查阅 [时区](../../../../advanced/time-zone) 文档。如果不指定，则使用 "Asia/Shanghai" 时区
 
+  - `load_parallelism`
+
+    导入并发度，默认为1。调大导入并发度会启动多个执行计划同时执行导入任务，加快导入速度。
+
 ### Example
 
 1. 从 HDFS 导入一批数据