Add a javadoc explanation for ProgressiveDownloader that explains that it tries…

Add a javadoc explanation for ProgressiveDownloader that explains that it tries to skip downloading already cached bytes.

download calls through to CacheUtil#cache, which handles skipping already downloaded bytes.

It is useful for callers to know that ProgressiveDownloader has this behavior - otherwise, they might reimplement the logic themselves.

I hope this behavior is something that can be part of the public API :)

PiperOrigin-RevId: 274202995

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

@@ -29,6 +29,12 @@ import java.util.concurrent.atomic.AtomicBoolean;
 
 /**
  * A downloader for progressive media streams.
+ *
+ * <p>The downloader attempts to download the entire media bytes referenced by a {@link Uri} into a
+ * cache as defined by {@link DownloaderConstructorHelper}. Callers can use the constructor to
+ * specify a custom cache key for the downloaded bytes.
+ *
+ * <p>The downloader will avoid downloading already-downloaded media bytes.
  */
 public final class ProgressiveDownloader implements Downloader {