feat(apple): Add docs for performanceV2 (#9109)

Add documentation for the performanceV2 option.

Co-authored-by: Liza Mock <[email protected]>

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

@@ -391,6 +391,22 @@ SentrySDK.reportFullyDisplayed()
 If the span finishes through the API, its `status` is set to `SpanStatus.OK`. If the span doesn't finish after 30 seconds, it will be finished by the SDK automatically, and its `status` will be set to `SpanStatus.DEADLINE_EXCEEDED`, also its duration will match the same of the `Time to initial display` span and the description will contain `Deadline Exceeded` suffix.
 If a call to `reportFullyDisplayed()` happens before the view controller appears, the reported time will be shifted to `Time to initial display` measured time.
 
+## Performance V2
+
+<Note>
+
+This feature is experimental and may have bugs.
+
+</Note>
+
+_(New in version 8.18.0)_
+
+We're working to update our **Performance** product offering in order to be able to provide better insights and highlight specific actions you can take to improve your mobile app's overall performance. The `performanceV2` option changes the following behavior:
+
+* The app start duration will now finish when the first frame is drawn instead of when the OS posts the `UIWindowDidBecomeVisibleNotification`.
+
+This change will be the default in the next major version.
+
 ## Opt Out
 
 You can opt out of all automatic instrumentations using the options: