Add usage instructions for multiple appid in the Apollo client (#5295)

* add multiple appid dosc

add multiple appid dosc

* fix some problem

* Apply suggestions from code review

---------

Co-authored-by: Jason Song <nobodyiam@gmail.com>

docs/en/client/java-sdk-user-guide.md

@@ -611,6 +611,17 @@ ConfigFile configFile = ConfigService.getConfigFile("test", ConfigFileFormat.XML
 String content = configFile.getContent();
 ```
 
+### 3.1.5 Read the configuration corresponding to multiple appid and their namespaces.(added in version 2.4.0)
+Specify the corresponding appid and namespace to retrieve the config, and then obtain the properties.
+```java
+String someAppId = "Animal";
+String somePublicNamespace = "CAT";
+Config config = ConfigService.getConfig(someAppId, somePublicNamespace);
+String someKey = "someKeyFromPublicNamespace";
+String someDefaultValue = "someDefaultValueForTheKey";
+String value = config.getProperty(someKey, someDefaultValue);
+```
+
 ## 3.2 Spring integration approach
 
 ### 3.2.1 Configuration
@@ -749,6 +760,18 @@ public class SomeAppConfig {
 public class AnotherAppConfig {}
 ```
 
+4.Support for multiple appid (added in version 2.4.0)
+```java
+// Added support for loading multiple appid their corresponding namespaces. 
+// Note that when using multiple appid, if there are keys that are the same, 
+// only the key from the prioritized loaded appid will be retrieved
+@Configuration
+@EnableApolloConfig(value = {"FX.apollo", "application.yml"},
+        multipleConfigs = {@MultipleConfig(appid = "ORDER_SERVICE", namespaces = {"ORDER.apollo"})}
+)
+public class SomeAppConfig {}
+```
+
 #### 3.2.1.3 Spring Boot integration methods (recommended)
 
 Spring Boot supports the above two integration methods in addition to configuration via application.properties/bootstrap.properties, which enables configuration to be injected at an earlier stage, such as scenarios that use `@ConditionalOnProperty` or have some spring-boot-starter needs to read the configuration to do something in the startup phase (e.g. [dubbo-spring-boot-project](https://github.com/apache/incubator-dubbo-spring-boot-project)). So for Spring Boot environment it is recommended to access Apollo (requires version 0.10.0 and above) by the following way.

docs/zh/client/java-sdk-user-guide.md

@@ -583,6 +583,17 @@ ConfigFile configFile = ConfigService.getConfigFile("test", ConfigFileFormat.XML
 String content = configFile.getContent();
 ```
 
+### 3.1.5 读取多AppId对应namespace的配置
+指定对应的AppId和namespace来获取Config，再获取属性
+```java
+String someAppId = "Animal";
+String somePublicNamespace = "CAT";
+Config config = ConfigService.getConfig(someAppId, somePublicNamespace);
+String someKey = "someKeyFromPublicNamespace";
+String someDefaultValue = "someDefaultValueForTheKey";
+String value = config.getProperty(someKey, someDefaultValue);
+```
+
 ## 3.2 Spring整合方式
 
 ### 3.2.1 配置
@@ -719,6 +730,17 @@ public class SomeAppConfig {
 public class AnotherAppConfig {}
 ```
 
+4.多appId的支持(新增于2.4.0版本)
+```java
+// 新增支持了多appId和对应namespace的加载，注意使用多appId的情况下，key相同的情况，只会取优先加载appId的那一个key
+@Configuration
+@EnableApolloConfig(value = {"FX.apollo", "application.yml"},
+        multipleConfigs = {@MultipleConfig(appid = "ORDER_SERVICE", namespaces = {"ORDER.apollo"})}
+)
+public class SomeAppConfig {}
+```
+
+
 #### 3.2.1.3 Spring Boot集成方式（推荐）
 
 Spring Boot除了支持上述两种集成方式以外，还支持通过application.properties/bootstrap.properties来配置，该方式能使配置在更早的阶段注入，比如使用`@ConditionalOnProperty`的场景或者是有一些spring-boot-starter在启动阶段就需要读取配置做一些事情（如[dubbo-spring-boot-project](https://github.com/apache/incubator-dubbo-spring-boot-project)），所以对于Spring Boot环境建议通过以下方式来接入Apollo(需要0.10.0及以上版本）。