feat(docs): clarify init mode for flutter (#14124)

Author: ioannisj

contents/docs/integrate/feature-flags-code/_snippets/feature-flags-code-flutter.mdx

@@ -22,6 +22,8 @@ if (await Posthog().getFeatureFlag('flag-key') == 'variant-key') { // replace 'v
 
 ### Ensuring flags are loaded before usage
 
+> To use the `onFeatureFlags` callback, you must [set up the SDK manually](#installation) by disabling the `com.posthog.posthog.AUTO_INIT` mode.
+
 Every time a user opens the app, we send a request in the background to fetch the feature flags that apply to that user. We store those flags in the storage.
 
 This means that for most screens, the feature flags are available immediately – **except for the first time a user visits**.