Better document how to use the login plugin message API

api/src/main/java/com/velocitypowered/api/event/connection/PreLoginEvent.java

@@ -21,6 +21,12 @@ import org.checkerframework.checker.nullness.qual.Nullable;
  * (for offline mode). Velocity will wait for this event to finish firing before proceeding further
  * with the login process, but you should try to limit the work done in any event that fires during
  * the login process.
+ *
+ * <p>
+ *   As of Velocity 3.1.0, you may cast the {@link InboundConnection} to a
+ *   {@link com.velocitypowered.api.proxy.LoginPhaseConnection} to allow a
+ *   proxy plugin to send login plugin messages to the client.
+ * </p>
  */
 @AwaitingEvent
 public final class PreLoginEvent implements ResultedEvent<PreLoginEvent.PreLoginComponentResult> {