fix docs and add java example (#2021)

mirai-console/docs/Extensions.md

@@ -32,6 +32,24 @@ object MyPlugin : KotlinPlugin( /* ... */ ) {
 
 ```
 
+```java
+// java
+public class MyPlugin extends JavaPlugin {
+    public MyPlugin() {
+        // ...
+    }
+
+    @Override
+    public void onLoad(PluginComponentStorage pcs) {
+        pcs.contributePermissionService(() -> { /* ... */ });
+        pcs.contributePluginLoader(() -> { /* ... */ });
+        pcs.contribute(ExtensionPoint, () -> { /* ... */ });
+    }
+}
+```
+
+
+
 ### 可用扩展
 
 查看 [extensions](../backend/mirai-console/src/extensions)。每个文件对应一个扩展。

mirai-console/docs/plugin/JVMPlugin.md

@@ -322,7 +322,7 @@ public final class JExample extends JavaPlugin {
     }
 
     @Override
-    public onEnable() {
+    public void onEnable() {
         if (getLogger().isInfoEnabled()) {
             getLogger().info("一条 INFO 级别的日志"); // 当日志被用户启用时才会执行
         }
@@ -444,7 +444,7 @@ public final class JExample extends JavaPlugin {
     }
 
     @Override
-    public onEnable() {
+    public void onEnable() {
         getScheduler().delayed(1000L, () -> System.out.println("一秒钟过去了。"));
     }
 }
@@ -475,7 +475,7 @@ public final class JExample extends JavaPlugin {
     }
 
     @Override
-    public onEnable() {
+    public void onEnable() {
         File dataFile = resolveDataFile("myDataFile.txt");
         File configFile = resolveConfigFile("myConfigFile.txt");
     }