Document that channelNameResourceId needs to be set

This is documented on the setter already, but it seems to make sense to do this in the constructor as well for clarity.

Issue: google/ExoPlayer#9550
PiperOrigin-RevId: 411675700

library/ui/src/main/java/com/google/android/exoplayer2/ui/PlayerNotificationManager.java

@@ -343,7 +343,9 @@ public class PlayerNotificationManager {
      *
      *
      * @param context The {@link Context}.
      * @param context The {@link Context}.
      * @param notificationId The id of the notification to be posted. Must be greater than 0.
      * @param notificationId The id of the notification to be posted. Must be greater than 0.
-     * @param channelId The id of the notification channel.
+     * @param channelId The id of the notification channel of an existing notification channel or of
+     *     the channel that should be automatically created. In the latter case, {@link
+     *     #setChannelNameResourceId(int)} needs to be called as well.
      */
      */
     public Builder(Context context, @IntRange(from = 1) int notificationId, String channelId) {
     public Builder(Context context, @IntRange(from = 1) int notificationId, String channelId) {
       checkArgument(notificationId > 0);
       checkArgument(notificationId > 0);