fix(apple): Clarify prewarmed app starts (#15110)

Clearly point out that the prewarmed app start tracing drops two spans
and remove the confusing part of not being able to reliably measuring
app starts.

Author: philipphofmann

docs/platforms/apple/common/tracing/instrumentation/automatic-instrumentation.mdx

@@ -124,9 +124,10 @@ Cold and warm start are Mobile Vitals, which you can learn about in the [full do
 
 ### Prewarmed App Start Tracing
 
-Starting with iOS 15, the system might [prewarm](https://developer.apple.com/documentation/uikit/app_and_environment/responding_to_the_launch_of_your_app/about_the_app_launch_sequence#3894431) your app by creating the process before the user opens it. In these cases, we won’t be able to reliably measure the app start. Enable the `enablePreWarmedAppStartTracing` feature to collect prewarmed app starts.
+Starting with iOS 15, the operating system might [prewarm](https://developer.apple.com/documentation/uikit/app_and_environment/responding_to_the_launch_of_your_app/about_the_app_launch_sequence#3894431) your app by creating the process before the user opens it. Enable the `enablePreWarmedAppStartTracing` feature to collect prewarmed app starts, which will be enabled by default in V9.
+
+When the OS prewarms your app process, it already does some of the initialization steps of a normal app start before the user taps the app icon. This can be anywhere from seconds to minutes or hours before your app is fully launched. Therefore, the SDK doesn't add the **Pre Runtime Init** and the **Runtime Init to Pre Main Initializers** spans. This approach shortens the app start duration, but it represents the time from when a user clicks the app icon to when the app is responsive.
 
-Once enabled, the SDK drops the first app start span if prewarming pauses. This approach shortens the app start duration, but it accurately represents the span of time from when a user clicks the app icon to when the app is responsive.
 With this feature, the SDK differentiates between four different app start types, which it adds to the app context of the transaction:
 
 - **Non-prewarmed cold start** (See _cold start_ definition above.)