hadoop: add doc for user and uid sync to avoid perm error (#340)

* hadoop: add doc for user and uid sync to avoid perm error

* hadoop: update

docs/en/hadoop_java_sdk.md

@@ -2,6 +2,8 @@
 
 JuiceFS provides [Hadoop-compatible FileSystem](https://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-common/filesystem/introduction.html) by Hadoop Java SDK to support variety of components in Hadoop ecosystem.
 
+**NOTICE: JuiceFS use local user uid mapping. So, you should sync all the needed users and their uids across the whole Hadoop Cluster to avoid permission error.**
+
 ## Hadoop Compatibility
 
 JuiceFS Hadoop Java SDK is compatible with Hadoop 2.x and Hadoop 3.x. As well as variety of components in Hadoop ecosystem.

docs/zh_cn/hadoop_java_sdk.md

@@ -2,6 +2,8 @@
 
 JuiceFS 提供兼容 HDFS 接口的 Java 客户端来支持 Hadoop 生态中的各种应用。
 
+**注意：由于 JuiceFS 使用本地的 user 和 uid 映射。因此，在分布式环境下使用，需要同步所有需要使用的 user 和 uid 到所有的 Hadoop 节点上，以避免权限问题。**
+
 ## Hadoop 兼容性
 
 JuiceFS Hadoop Java SDK 同时兼容 Hadoop 2.x 以及 Hadoop 3.x 环境，以及 Hadoop 生态中的各种主流组件。