update doc

Author: buenaflor

platform-includes/enriching-events/attach-screenshots/flutter.mdx

@@ -21,6 +21,8 @@ You can filter your screenshots by using the `beforeScreenshot` callback which i
 
 If the callback returns `false`, the screenshot will not be attached.
 
+By default, the callback returns `true` which means that all screenshots are attached.
+
 ```dart
 await SentryFlutter.init((options) {
   options.beforeScreenshot = (event, {hint}) {