more javadoc about how the download URL is used as a unique ID

app/src/main/java/org/fdroid/fdroid/ProgressListener.java

@@ -7,6 +7,15 @@ import java.net.URL;
  * updates, APKs, etc). This also keeps this class pure Java so that classes
  * that use {@code ProgressListener} can be tested on the JVM, without requiring
  * an Android device or emulator.
+ * <p/>
+ * The full URL of a download is used as the unique identifier throughout
+ * F-Droid.  I can take a few forms:
+ * <ul>
+ * <li>{@link URL} instances
+ * <li>{@link android.net.Uri} instances
+ * <li>{@code String} instances, i.e. {@link URL#toString()}
+ * <li>{@code int}s, i.e. {@link String#hashCode()}
+ * </ul>
  */
 public interface ProgressListener {