Remove documentation on calling startDownloads

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=194961264

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

@@ -47,9 +47,6 @@ import java.util.concurrent.CopyOnWriteArraySet;
  * <p>A download manager instance must be accessed only from the thread that created it, unless that
  * thread does not have a {@link Looper}. In that case, it must be accessed only from the
  * application's main thread. Registered listeners will be called on the same thread.
- *
- * <p>By default download tasks are stopped, so {@link #startDownloads()} must be called to start
- * them.
  */
 public final class DownloadManager {
 

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

@@ -250,7 +250,9 @@ public abstract class DownloadService extends Service {
 
   /**
    * Returns a {@link DownloadManager} to be used to downloaded content. Called only once in the
-   * life cycle of the service.
+   * life cycle of the service. The service will call {@link DownloadManager#startDownloads()} and
+   * {@link DownloadManager#stopDownloads} as necessary when requirements returned by {@link
+   * #getRequirements()} are met or stop being met.
    */
   protected abstract DownloadManager getDownloadManager();