react-native: perf auto instr (#4511)

Co-authored-by: Bruno Garcia <bruno@brunogarcia.com>
Co-authored-by: Manoel Aranda Neto <5731772+marandaneto@users.noreply.github.com>

Author: bruno-garcia

src/platforms/react-native/performance/instrumentation/automatic-instrumentation.mdx

@@ -22,6 +22,12 @@ export default Sentry.wrap(App);
 
 ### Enable Routing Instrumentation
 
+<Note>
+
+When no routing instrumentation is used, a transaction for `App Start` is automatically captured. However, that transaction stops being sent when one of the routing integrations below is added. Instead, the `App Start` information is included as a span in a transaction captured by the routing instrumentation.
+
+</Note>
+
 We currently provide three routing instrumentations out of the box to instrument route changes for:
 
 - [React Navigation](#react-navigation)