VoiceReceiverAdapter.java: add default missing doc

[christolis]

Why

Sonarlint wants every public class to have a JavaDoc.

[tj-wazei]

• {@link #onVoiceUpdate(GuildVoiceUpdateEvent)} like the other provided methods can be overridden
• if desired. The default implementation is empty, the adapter will not react to such events.

Seems unnecessary but approved anyway, the intent is there.

But if you want to make it better...

This class follows the adapter pattern: it provides empty (no-op) implementations for all {@link VoiceReceiver} callback methods, allowing subclasses to override only the events they are interested in.