Docs: Simplify AppDelegate Info.plist option in README

Further refines the documentation for the `FirebaseAppDelegateClassName`
Info.plist key feature on iOS.

- The explanation in `release_build_files/readme.md` under "Specifying Your
  AppDelegate Class Directly (iOS)" has been made more concise and
  user-focused, removing internal implementation details.
- The corresponding release note for version 12.9.0 has also been
  simplified to match this approach.

This change aims to make the documentation easier for developers to
understand by focusing on the action and benefit rather than Firebase
internal mechanisms.

Author: google-labs-jules[bot]

app/src/util_ios.mm

@@ -239,7 +239,6 @@ + (void)load {
 namespace firebase {
 namespace util {
 
-// This is the ORIGINAL implementation of RunOnAppDelegateClasses
 void RunOnAppDelegateClasses(void (^block)(Class)) {
   if (g_seen_delegate_classes_count > 0) {
     NSLog(@"Firebase: RunOnAppDelegateClasses executing block for %d already seen delegate class(es).",

release_build_files/readme.md

@@ -537,9 +537,30 @@ addition to any you may have implemented.
 
 The Firebase Cloud Messaging library needs to attach
 handlers to the application delegate using method swizzling. If you are using
-these libraries, at load time, Firebase will identify your `AppDelegate` class
-and swizzle the required methods onto it, chaining a call back to your existing
-method implementation.
+these libraries, at load time, Firebase will typically identify your `AppDelegate`
+class and swizzle the required methods onto it.
+
+#### Specifying Your AppDelegate Class Directly (iOS)
+
+For a more direct approach, or if you encounter issues with the default
+method swizzling, you can explicitly tell Firebase which class is your
+application's `AppDelegate`. To do this, add the `FirebaseAppDelegateClassName`
+key to your app's `Info.plist` file:
+
+*   **Key:** `FirebaseAppDelegateClassName`
+*   **Type:** `String`
+*   **Value:** Your AppDelegate's class name (e.g., `MyCustomAppDelegate`)
+
+**Example `Info.plist` entry:**
+```xml
+<key>FirebaseAppDelegateClassName</key>
+<string>MyCustomAppDelegate</string>
+```
+
+If this key is provided with a valid class name, Firebase will use that class
+directly for its AppDelegate-related interactions. If the key is not present,
+is invalid, or the class is not found, Firebase will use its standard method
+swizzling approach.
 
 ### Custom Android Build Systems
 
@@ -654,6 +675,14 @@ workflow use only during the development of your app, not for publicly shipping
 code.
 
 ## Release Notes
+### 12.9.0 (Upcoming)
+-   Changes
+    - iOS: Added an option to explicitly specify your app's `AppDelegate` class
+      name via the `FirebaseAppDelegateClassName` key in `Info.plist`. This
+      provides a more direct way for Firebase to interact with your specified
+      AppDelegate. See "Platform Notes > iOS Method Swizzling >
+      Specifying Your AppDelegate Class Directly (iOS)" for details.
+
 ### 12.8.0
 -   Changes
     - General (iOS): Update to Firebase Cocoapods version 11.14.0.