update doc for debug mode

Author: yyx990803

source/api/global-api.md

@@ -34,7 +34,15 @@ You can modify them directly, for example:
 Vue.config.debug = true // turn on debugging mode
 ```
 
-Example for changing interpolation delimiters:
+**Debug Mode**
+
+When `Vue.config.debug` is set to true, Vue will automatically use synchronous mode and throw a `debugger` statement when there is a warning. This enables the user to inspect the full stack trace in browser dev tools.
+
+<p class="tip">debug mode is not available in the minified production builds.</p>
+
+**Changing Delimiters**
+
+When the delimiters are set for text interpolation, the delimiters for HTML interpolation will be generated by adding one outer-most symbol on both sides:
 
 ``` js
 Vue.config.delimiters = ['(%', '%)']