[RUM-8832] Clarify SDK and RUM initialization timing requirements in setup.md (#30551)

Author: ncreated

content/en/real_user_monitoring/mobile_and_tv_monitoring/ios/setup.md

@@ -131,7 +131,11 @@ For more information about setting up a client token, see the [Client token docu
 
 ### Step 3 - Initialize the library
 
-In the initialization snippet, set an environment name, service name, and version number. In the examples below, `app-name` specifies the variant of the application that generates data.
+In the initialization snippet, set an environment name, service name, and client token.
+
+The SDK should be initialized as early as possible in the app lifecycle, specifically in the `AppDelegate`'s `application(_:didFinishLaunchingWithOptions:)` callback. This ensures all measurements, including application startup duration, are captured correctly. For apps built with SwiftUI, you can use `@UIApplicationDelegateAdaptor` to hook into the `AppDelegate`.
+
+<div class="alert alert-warning">Initializing the SDK elsewhere (for example later during view loading) may result in inaccurate or missing telemetry, especially around app startup performance.</div>
 
 For more information, see [Using Tags][5].
 
@@ -402,8 +406,8 @@ For example, if the current tracking consent is `.pending`:
 
 ### Step 4 - Start sending data
 
-#### Initialize the Datadog Monitor
-Configure and register the Datadog Monitor. You only need to do it once, usually in your `AppDelegate` code:
+#### Enable RUM
+Configure and start RUM. This should be done once and as early as possible, specifically in your `AppDelegate`:
 
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