modify baseContext

Change-Id: I511f26472248a1cf5abd5e7d8e9f63e108cd28cf
Signed-off-by: NHuangXW <huangxinwei4@huawei.com>

zh-cn/application-dev/reference/apis/js-apis-inner-application-baseContext.md

 # BaseContext
 
-BaseContext抽象类用于表示继承的子类Context是Stage模型还是FA模型。
+BaseContext抽象类用于表示继承的子类Context是Stage模型还是FA模型，是所有Context类型的父类。
 
 > **说明：**
 >
@@ -14,10 +14,15 @@ BaseContext抽象类用于表示继承的子类Context是Stage模型还是FA模
 
 **示例：**
 
-  ```ts
-  class MyContext extends BaseContext {
-      constructor(stageMode) {
-          this.stageMode = stageMode;
-      }
+以Stage模型为例，用户可通过UIAbilityContext访问stageMode字段。
+
+```ts
+import UIAbility from '@ohos.app.ability.UIAbility';
+
+class MainAbility extends UIAbility {
+    onCreate(want, launchParam) {
+        // MainAbility onCreate, isStageMode: true
+        console.log("MainAbility onCreate, isStageMode: " + this.context.stageMode);
     }
-  ```
\ No newline at end of file
+}
+```