Enhance the java doc of dubbo-container module (#3437)

Fixes #2994

dubbo-container/dubbo-container-api/src/main/java/org/apache/dubbo/container/Container.java

@@ -25,12 +25,12 @@ import org.apache.dubbo.common.extension.SPI;
 public interface Container {
 
     /**
-     * start.
+     * start method to load the container.
      */
     void start();
 
     /**
-     * stop.
+     * stop method to unload the container.
      */
     void stop();
 

dubbo-container/dubbo-container-api/src/main/java/org/apache/dubbo/container/Main.java

@@ -33,6 +33,8 @@ import java.util.concurrent.locks.ReentrantLock;
 
 /**
  * Main. (API, Static, ThreadSafe)
+ *
+ * This class is entry point loading containers.
  */
 public class Main {
 

dubbo-container/dubbo-container-log4j/src/main/java/org/apache/dubbo/container/log4j/Log4jContainer.java

@@ -30,6 +30,8 @@ import java.util.Properties;
 
 /**
  * Log4jContainer. (SPI, Singleton, ThreadSafe)
+ *
+ * The container class implementation for Log4j
  */
 public class Log4jContainer implements Container {
 

dubbo-container/dubbo-container-logback/src/main/java/org/apache/dubbo/container/logback/LogbackContainer.java

@@ -31,6 +31,8 @@ import org.slf4j.LoggerFactory;
 
 /**
  * LogbackContainer. (SPI, Singleton, ThreadSafe)
+ *
+ * The container class implementation for Logback
  */
 public class LogbackContainer implements Container {
 

dubbo-container/dubbo-container-spring/src/main/java/org/apache/dubbo/container/spring/SpringContainer.java

@@ -26,6 +26,8 @@ import org.springframework.context.support.ClassPathXmlApplicationContext;
 
 /**
  * SpringContainer. (SPI, Singleton, ThreadSafe)
+ *
+ * The container class implementation for Spring
  */
 public class SpringContainer implements Container {