Update screenshot-endpoint.mdx

add what the default viewport is

Author: kathayl

src/content/docs/browser-rendering/rest-api/screenshot-endpoint.mdx

@@ -51,6 +51,9 @@ For more options to control the final screenshot, like `clip`, `captureBeyondVie
 
 :::note
 The `quality` parameter is not compatible with the default `.png` format and will return a 400 error. If you set `quality`, you must also set `type` to `.jpeg` or another supported format.
+
+By default, the browser viewport is set to **1920×1080**. You can override the default via request options.
+
 :::
 
 ## Advanced usage