[IOTDB-2313][REST API Doc] how to avoid GC overhead (#4926)

Co-authored-by: NSteve Yurong Su <rong@apache.org>

docs/UserGuide/API/RestService.md

@@ -22,13 +22,26 @@
 ## RESTful Services  
 IoTDB's RESTful services can be used for query, write, and management operations, using the OpenAPI standard to define interfaces and generate frameworks.
 
+### Enable RESTful Services
 
+RESTful services are disabled by default.
+
+* Developer
+
+  Find the `IoTDBrestServiceConfig` class under `org.apache.iotdb.db.conf.rest` in the sever module, and modify `enableRestService=true`.
+
+* User
+
+  Find the `conf/iotdb.properties` file under the IoTDB installation directory and set `enable_rest_service` to `true` to enable the module.
+
+  ```properties
+  enable_rest_service=true
+  ```
 
 ### Authentication
 RESTful services use the basic authentication. Each URL request needs to carry `'Authorization': 'Basic ' + base64.encode(username + ':' + password)`.
 
 
-
 ### Interface
 
 #### ping
@@ -96,6 +109,8 @@ Response parameters:
 
 **Examples:**
 
+Tip: Statements like `select * from root.xx.**` are not recommended because those statements may cause OOM.
+
 **Expression query**
 
 ```shell

docs/UserGuide/Ecosystem Integration/Grafana Plugin.md

@@ -213,6 +213,8 @@ Here are some examples of valid CONTROL content:
 *  `group by ([0, 20), 2ms, 3ms), level=1`
 
 
+Tip: Statements like `select * from root.xx.**` are not recommended because those statements may cause OOM.
+
 #### Support for variables and template functions
 
 This plugin supports Grafana's variables and templates (https://grafana.com/docs/grafana/v7.0/variables/) feature.

docs/zh/UserGuide/API/RestService.md

@@ -22,7 +22,19 @@
 ## RESTful 服务
 IoTDB 的 RESTful 服务可用于查询、写入和管理操作，它使用 OpenAPI 标准来定义接口并生成框架。
 
-
+### 开启RESTful 服务
+RESTful 服务默认情况是关闭的
+ * 开发者  
+   
+   找到sever模块中`org.apache.iotdb.db.conf.rest` 下面的`IoTDBRestServiceConfig`类，修改`enableRestService=true`即可。
+ 
+ * 使用者  
+   
+   找到IoTDB安装目录下面的`conf/iotdb.properties`文件，将 `enable_rest_service` 设置为 `true` 以启用该模块。
+    
+   ```properties
+    enable_rest_service=true
+    ```
 
 ### 鉴权
 RESTful 服务使用了基础（basic）鉴权，每次 URL 请求都需要在 header 中携带 `'Authorization': 'Basic ' + base64.encode(username + ':' + password)`。
@@ -96,6 +108,8 @@ query 接口可以用于处理数据查询和元数据查询。
 
 请求示例如下所示：
 
+提示:为了避免OOM问题，不推荐使用select * from root.xx.** 这种查找方式。
+
 请求示例 表达式查询:
 ```shell
 curl -H "Content-Type:application/json" -H "Authorization:Basic cm9vdDpyb290" -X POST --data '{"sql":"select s3, s4, s3 + 1 from root.sg27 limit 2"}' http://127.0.0.1:18080/rest/v1/query

docs/zh/UserGuide/Ecosystem Integration/Grafana Plugin.md

@@ -220,7 +220,7 @@ CONTROL 输入框为非必须填写项目，填写内容应当是控制查询类
 *  `group by ((2017-11-01T00:00:00, 2017-11-07T23:00:00], 1d), level=1`
 *  `group by ([0, 20), 2ms, 3ms), level=1`
 
-
+提示：为了避免OOM问题，不推荐使用select * from root.xx.** 这种语句在Grafana plugin中使用。
 
 #### 变量与模板功能的支持