Add clearer comment about headless Onyx init

src/setup/index.js

@@ -7,10 +7,19 @@ import platformSetup from './platformSetup';
 import {canCaptureOnyxMetrics} from '../libs/canCaptureMetrics';
 
 export default function () {
-    // Initialize the Onyx store when the app loads for the first time.
-    // Note: This Onyx initialization has been very intentionally placed outside of the React lifecycle of the main App component.
-    //       This enables us to use Onyx in a headless JS context (i.e: to update Onyx data in response to a push notification received when the app is completely killed).
-    //       More info here: https://github.com/transistorsoft/react-native-background-fetch/issues/165#issuecomment-514255928
+    /*
+     * Initialize the Onyx store when the app loads for the first time.
+     *
+     * Note: This Onyx initialization has been very intentionally placed completely outside of the React lifecycle of the main App component.
+     *
+     * To understand why we must do this, you must first understand that a typical React Native Android application consists of an Application and an Activity.
+     * The project root's index.js runs in the Application, but the main RN `App` component + UI runs in a separate Activity, spawned when you call AppRegistry.registerComponent.
+     * When an application launches in a headless JS context (i.e: when woken from a killed state by a push notification), only the Application is available, but not the UI Activity.
+     * This means that in a headless context NO REACT CODE IS EXECUTED, and none of your components will mount.
+     *
+     * However, we still need to use Onyx to update the underlying app data from the headless JS context.
+     * Therefore it must be initialized completely outside the React component lifecycle.
+     */
     Onyx.init({
         keys: ONYXKEYS,
         safeEvictionKeys: [ONYXKEYS.COLLECTION.REPORT_ACTIONS],