doc: document restrictions for BindButtonEvent (MovingBlocks#5079)

While working on Terasology/FlexiblePathfinding#8 we noticed that there seem to be issues with sending events extending from BindButtonEvent programmatically (without the physical button press).

To avoid any future confusion (or at least find the issue sooner) I'm adding a brief docstring to the class stating it's intended use and that one should NOT use it directly.

engine/src/main/java/org/terasology/engine/input/BindButtonEvent.java

@@ -1,4 +1,4 @@
-// Copyright 2021 The Terasology Foundation
+// Copyright 2022 The Terasology Foundation
 // SPDX-License-Identifier: Apache-2.0
 
 package org.terasology.engine.input;
@@ -7,6 +7,15 @@
 import org.terasology.engine.input.events.ButtonEvent;
 import org.terasology.input.ButtonState;
 
+/**
+ * An event triggered by auto-registered (physical) buttons when they are pressed.
+ * <p>
+ * To be used in combination with {@link RegisterBindButton} and {@link DefaultBinding}.
+ * <p>
+ * Classes extending this usually follow the naming pattern {@code &#60;Name&#62;Button}.
+ * <p>
+ * <b>NOTE:</b> DO NOT USE DIRECTLY!
+ */
 public class BindButtonEvent extends ButtonEvent {
 
     private SimpleUri id;