Fix docs on playUntilStartOfMediaItem to match playUntilPosition

The behaviour and docs of `playUntilPosition` were changed in
00c7a9b.

This change also affects `playUntilStartOfMediaItem` (since it delegates
to `playUntilPosition`), so the same doc change should also be made
here.

#minor-release

PiperOrigin-RevId: 607364897

libraries/test_utils_robolectric/src/main/java/androidx/media3/test/utils/robolectric/TestPlayerRunHelper.java

@@ -340,8 +340,12 @@ public static void playUntilPosition(ExoPlayer player, int mediaItemIndex, long
 
   /**
    * Calls {@link Player#play()}, runs tasks of the main {@link Looper} until the {@code player}
-   * reaches the specified media item or a playback error occurs, and then pauses the {@code
-   * player}.
+   * reaches the specified media item or a playback error occurs.
+   *
+   * <p>The playback thread is automatically blocked from making further progress after reaching the
+   * media item and will only be unblocked by other {@code run/playUntil...} methods, custom {@link
+   * RobolectricUtil#runMainLooperUntil} conditions or an explicit {@link
+   * ThreadTestUtil#unblockThreadsWaitingForProgressOnCurrentLooper()} on the main thread.
    *
    * <p>If a playback error occurs it will be thrown wrapped in an {@link IllegalStateException}.
    *