Update server document

docs/cn/server.md

@@ -1017,7 +1017,7 @@ public:
 
 Server默认使用`DefaultRpcPBMessageFactory`。它是一个简单的工厂类，通过`new`来创建请求/响应message和`delete`来销毁请求/响应message。
 
-如果用户希望自定义创建销毁机制，可以实现`RpcPBMessages`（请求/响应message的封装）和`RpcPBMessageFactory`（工厂类），并通过`ServerOptions.rpc_pb_message_factory`。
+如果用户希望自定义创建销毁机制，可以实现`RpcPBMessages`（请求/响应message的封装）和`RpcPBMessageFactory`（工厂类），并设置`ServerOptions.rpc_pb_message_factory`为自定义的`RpcPBMessageFactory`。注意：server启动后，server拥有了`RpcPBMessageFactory`的所有权。
 
 接口如下：
 
@@ -1051,6 +1051,11 @@ public:
 
 ### Protobuf arena
 
+Protobuf arena是一种Protobuf message内存管理机制，有着提高内存分配效率、减少内存碎片、对缓存友好等优点。详细信息见[C++ Arena Allocation Guide](https://protobuf.dev/reference/cpp/arenas/)。
+
+如果用户希望使用protobuf arena来管理Protobuf message内存，可以设置`ServerOptions.rpc_pb_message_factory = brpc::GetArenaRpcPBMessageFactory();`，使用默认的`start_block_size`（256 bytes）和`max_block_size`（8192 bytes）来创建arena。用户可以调用`brpc::GetArenaRpcPBMessageFactory<StartBlockSize, MaxBlockSize>();`自定义arena大小。
+
+注意：从Protobuf v3.14.0开始，[默认开启arena](https://github.com/protocolbuffers/protobuf/releases/tag/v3.14.0https://github.com/protocolbuffers/protobuf/releases/tag/v3.14.0)。但是Protobuf v3.14.0之前的版本，用户需要再proto文件中加上选项：`option cc_enable_arenas = true;`，所以为了兼容性，可以统一都加上该选项。
 
 # FAQ
 

docs/en/server.md

@@ -1010,9 +1010,9 @@ public:
 
 ## RPC Protobuf message factory
 
-`DefaultRpcPBMessageFactory' is used at server-side by default. It is a simple factory class that uses `new' to create request/response messages and `delete' to destroy request/response messages.
+`DefaultRpcPBMessageFactory' is used at server-side by default. It is a simple factory class that uses `new' to create request/response messages and `delete' to destroy request/response messages. Currently, the baidu_std protocol and HTTP protocol support this feature.
 
-Users can implement `RpcPBMessages' (encapsulation of request/response message) and `RpcPBMessageFactory' (factory class) to customize the creation and destruction mechanism of protobuf message, and then set to `ServerOptions.rpc_pb_message_factory'.
+Users can implement `RpcPBMessages' (encapsulation of request/response message) and `RpcPBMessageFactory' (factory class) to customize the creation and destruction mechanism of protobuf message, and then set to `ServerOptions.rpc_pb_message_factory`. Note: After the server is started, the server owns the `RpcPBMessageFactory`.
 
 The interface is as follows:
 
@@ -1043,6 +1043,14 @@ public:
 };
 ```
 
+### Protobuf arena
+
+Protobuf arena is a Protobuf message memory management mechanism with the advantages of improving memory allocation efficiency, reducing memory fragmentation, and being cache-friendly. For more information, see [C++ Arena Allocation Guide](https://protobuf.dev/reference/cpp/arenas/).
+
+Users can set `ServerOptions.rpc_pb_message_factory = brpc::GetArenaRpcPBMessageFactory();` to manage Protobuf message memory,  with the default `start_block_size` (256 bytes) and `max_block_size` (8192 bytes). Alternatively, users can use `brpc::GetArenaRpcPBMessageFactory<StartBlockSize, MaxBlockSize>();` to customize the arena size.
+
+Note: Since Protocol Buffers v3.14.0, Arenas are now unconditionally enabled. However, for versions prior to Protobuf v3.14.0, users need to add the option `option cc_enable_arenas = true;` to the proto file. so for compatibility, this option can be added uniformly.
+
 # FAQ
 
 ### Q: Fail to write into fd=1865 [email protected]:54742@8230: Got EOF