CenterQuad: new class javadoc (#1684)

* CenterQuad:  rewrote the class javadoc from scratch

* CenterQuad: improve the javadoc style

jme3-vr/src/main/java/com/jme3/scene/CenterQuad.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2009-2010 jMonkeyEngine
+ * Copyright (c) 2009-2021 jMonkeyEngine
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -36,11 +36,15 @@ import com.jme3.scene.Mesh;
 import com.jme3.scene.VertexBuffer.Type;
 
 /**
- * <code>Quad</code> represents a rectangular plane in space
- * defined by 4 vertices. The quad's lower-left side is contained
- * at the local space origin (0, 0, 0), while the upper-right
- * side is located at the width/height coordinates (width, height, 0).
- * 
+ * A static, indexed, Triangles-mode mesh for an axis-aligned rectangle in the
+ * X-Y plane.
+ *
+ * <p>The rectangle extends from (-width/2, -height/2, 0) to
+ * (width/2, height/2, 0) with normals set to (0,0,1).
+ *
+ * <p>This differs from {@link com.jme3.scene.shape.Quad} because it puts
+ * (0,0,0) at the rectangle's center instead of in a corner.
+ *
  * @author Kirill Vainer
  */
 public class CenterQuad extends Mesh {