[8.x] Update ShouldBroadcast::broadcastOn docblock to return strings (#38444)

* Update ShouldBroadcast docblock

* Update ShouldBroadcast.php

Co-authored-by: Taylor Otwell <[email protected]>

Author: sebastiandedeyne

src/Illuminate/Contracts/Broadcasting/ShouldBroadcast.php

@@ -7,7 +7,7 @@ interface ShouldBroadcast
     /**
      * Get the channels the event should broadcast on.
      *
-     * @return \Illuminate\Broadcasting\Channel|\Illuminate\Broadcasting\Channel[]
+     * @return \Illuminate\Broadcasting\Channel|\Illuminate\Broadcasting\Channel[]|string[]|string
      */
     public function broadcastOn();
 }