[fenix] For https://github.com/mozilla-mobile/fenix/issues/8803 - review: Add clarifying comments to onAppInit capture methods.

app/src/main/java/org/mozilla/fenix/FenixApplication.kt

@@ -374,6 +374,8 @@ open class FenixApplication : LocaleAwareApplication() {
     protected fun recordOnInit() {
         // This gets called by more than one process. Ideally we'd only run this in the main process
         // but the code to check which process we're in crashes because the Context isn't valid yet.
-        StartupTimeline.onApplicationInit()
+        //
+        // This method is not covered by our internal crash reporting: be very careful when modifying it.
+        StartupTimeline.onApplicationInit() // DO NOT MOVE ANYTHING ABOVE HERE: the timing is critical.
     }
 }

app/src/main/java/org/mozilla/fenix/perf/StartupTimeline.kt

@@ -46,6 +46,8 @@ object StartupTimeline {
 
     fun onApplicationInit() {
         // This gets called from multiple processes: don't do anything expensive. See call site for details.
+        //
+        // This method also gets called multiple times if there are multiple Application implementations.
         frameworkStartMeasurement.onApplicationInit()
     }