Add comment about broadcast receiver

app/src/main/java/org/schabi/newpipe/player/Player.java

@@ -663,6 +663,12 @@ public final class Player implements PlaybackListener, Listener {
     //////////////////////////////////////////////////////////////////////////*/
     //region Broadcast receiver
 
+    /**
+     * This function prepares the broadcast receiver and is called only in the constructor.
+     * Therefore if you want any PlayerUi to receive a broadcast action, you should add it here,
+     * even if that player ui might never be added to the player. In that case the received
+     * broadcast would not do anything.
+     */
     private void setupBroadcastReceiver() {
         if (DEBUG) {
             Log.d(TAG, "setupBroadcastReceiver() called");

app/src/main/java/org/schabi/newpipe/player/ui/PlayerUi.java

@@ -55,6 +55,10 @@ public abstract class PlayerUi {
     public void onFragmentListenerSet() {
     }
 
+    /**
+     * If you want to register new broadcast actions to receive here, add them to
+     * {@link Player#setupBroadcastReceiver()}.
+     */
     public void onBroadcastReceived(final Intent intent) {
     }