More documentation

Author: tejasd

firebase-perf/src/main/java/com/google/firebase/perf/metrics/AppStartTrace.java

@@ -591,8 +591,9 @@ public static boolean isScreenOn(Context appContext) {
   /**
    * We use StartFromBackgroundRunnable to detect if app is started from background or foreground.
    * If app is started from background, we do not generate AppStart trace. This runnable is posted
-   * to main UI thread from FirebasePerfEarly. If app is started from background, this runnable will
-   * be executed earlier than 100ms of any activity's onCreate() method.
+   * to main UI thread from FirebasePerfEarly. If `onActivityCreate` has never been called, we
+   * record the timestamp - which allows `onActivityCreate` to determine whether it was a background
+   * app start or not.
    */
   public static class StartFromBackgroundRunnable implements Runnable {
     private final AppStartTrace trace;