Add mutual links to cn/en in client.md

docs/cn/client.md

+[English version](../en/client.md)
+
 # 示例程序
 
 Echo的[client端代码](https://github.com/brpc/brpc/blob/master/example/echo_c++/client.cpp)。

docs/en/client.md

+[中文版](../cn/client.md)
+
 # Example
 
 [client-side code](https://github.com/brpc/brpc/blob/master/example/echo_c++/client.cpp) of echo.
@@ -11,7 +13,7 @@
 - No class named brpc::Client.
 
 # Channel
-Client-side sends requests. It's called [Channel](https://github.com/brpc/brpc/blob/master/src/brpc/channel.h) rather than "Client" in brpc. A channel represents a communication line to one server or multiple servers, which can be used for calling services.
+Client-side of RPC sends requests. It's called [Channel](https://github.com/brpc/brpc/blob/master/src/brpc/channel.h) rather than "Client" in brpc. A channel represents a communication line to one server or multiple servers, which can be used for calling services.
 
 A Channel can be **shared by all threads** in the process. Yon don't need to create separate Channels for each thread, and you don't need to synchronize Channel.CallMethod with lock. However creation and destroying of Channel is **not** thread-safe,  make sure the channel is initialized and destroyed only by one thread.