docs: add info about the `{open}` attr for `details` container (#4067)

Co-authored-by: Divyansh Singh <40380293+brc-dd@users.noreply.github.com>

docs/en/guide/markdown.md

@@ -187,7 +187,7 @@ You may set custom title by appending the text right after the "type" of the con
 Danger zone, do not proceed
 :::
 
-::: details Click me to view the code
+::: details Click me to toggle the code
 ```js
 console.log('Hello, VitePress!')
 ```
@@ -200,7 +200,7 @@ console.log('Hello, VitePress!')
 Danger zone, do not proceed
 :::
 
-::: details Click me to view the code
+::: details Click me to toggle the code
 ```js
 console.log('Hello, VitePress!')
 ```
@@ -225,6 +225,28 @@ export default defineConfig({
 })
 ```
 
+### Additional Attributes
+
+You can add additional attributes to the custom containers. We use [markdown-it-attrs](https://github.com/arve0/markdown-it-attrs) for this feature, and it is supported on almost all markdown elements. For example, you can set the `open` attribute to make the details block open by default:
+
+**Input**
+
+````md
+::: details Click me to toggle the code {open}
+```js
+console.log('Hello, VitePress!')
+```
+:::
+````
+
+**Output**
+
+::: details Click me to toggle the code {open}
+```js
+console.log('Hello, VitePress!')
+```
+:::
+
 ### `raw`
 
 This is a special container that can be used to prevent style and router conflicts with VitePress. This is especially useful when you're documenting component libraries. You might also wanna check out [whyframe](https://whyframe.dev/docs/integrations/vitepress) for better isolation.