Dev guide: Start updating the download page

PiperOrigin-RevId: 329719402

extensions/jobdispatcher/README.md

 # ExoPlayer Firebase JobDispatcher extension #
 
-**DEPRECATED - Please use [WorkManager extension][] or [PlatformScheduler][]
-instead.**
+**This extension is deprecated. Use the [WorkManager extension][] instead.**
 
 This extension provides a Scheduler implementation which uses [Firebase JobDispatcher][].
 
 [WorkManager extension]: https://github.com/google/ExoPlayer/blob/release-v2/extensions/workmanager/README.md
-[PlatformScheduler]: https://github.com/google/ExoPlayer/blob/release-v2/library/core/src/main/java/com/google/android/exoplayer2/scheduler/PlatformScheduler.java
 [Firebase JobDispatcher]: https://github.com/firebase/firebase-jobdispatcher-android
 
 ## Getting the extension ##

library/core/src/main/java/com/google/android/exoplayer2/offline/DownloadHelper.java

@@ -77,7 +77,7 @@ import org.checkerframework.checker.nullness.qual.RequiresNonNull;
  * <p>A typical usage of DownloadHelper follows these steps:
  *
  * <ol>
- *   <li>Build the helper using one of the {@code forXXX} methods.
+ *   <li>Build the helper using one of the {@code forMediaItem} methods.
  *   <li>Prepare the helper using {@link #prepare(Callback)} and wait for the callback.
  *   <li>Optional: Inspect the selected tracks using {@link #getMappedTrackInfo(int)} and {@link
  *       #getTrackSelections(int, int)}, and make adjustments using {@link