Add is_local paramter description (#8893)

* add is_local paramter description

* update

* update by comment

doc/v2/howto/cluster/cmd_argument_cn.md

@@ -71,6 +71,13 @@ paddle.init(
 - trainer_id：**必选，默认0**，每个trainer的唯一ID，从0开始的整数
 - pservers：**必选，默认127.0.0.1**，当前训练任务启动的pserver的IP列表，多个IP使用“,”隔开
 
+```python
+trainer = paddle.trainer.SGD(..., is_local=False)
+```
+
+参数说明
+
+- is_local: **必选, 默认True**, 是否使用PServer更新参数
 
 ## 准备数据集
 

doc/v2/howto/cluster/cmd_argument_en.md

@@ -73,6 +73,14 @@ Parameter Description
 - trainer_id: **required, default 0**, ID for every trainer, start from 0.
 - pservers: **required, default 127.0.0.1**, list of IPs of parameter servers, separated by ",".
 
+```python
+trainer = paddle.trainer.SGD(..., is_local=False)
+```
+
+Parameter Description
+
+- is_local: **required, default True**, whether update parameters by PServer.
+
 ## Prepare Training Dataset
 
 Here's some example code [prepare.py](https://github.com/PaddlePaddle/Paddle/tree/develop/doc/howto/usage/cluster/src/word2vec/prepare.py), it will download public `imikolov` dataset and split it into multiple files according to job parallelism(trainers count). Modify `SPLIT_COUNT` at the begining of `prepare.py` to change the count of output files.