Improve Memento Javadoc

memento/src/main/java/com/iluwatar/memento/App.java

@@ -4,7 +4,20 @@ import java.util.Stack;
 
 /**
  * 
- * Memento pattern is for storing and restoring object state. The object ({@link Star})
+ * The Memento pattern is a software design pattern that provides the ability to restore 
+ * an object to its previous state (undo via rollback).
+ * <p>
+ * The Memento pattern is implemented with three objects: the originator, a caretaker and 
+ * a memento. The originator is some object that has an internal state. The caretaker is 
+ * going to do something to the originator, but wants to be able to undo the change. The 
+ * caretaker first asks the originator for a memento object. Then it does whatever operation 
+ * (or sequence of operations) it was going to do. To roll back to the state before the 
+ * operations, it returns the memento object to the originator. The memento object itself 
+ * is an opaque object (one which the caretaker cannot, or should not, change). When using 
+ * this pattern, care should be taken if the originator may change other objects or 
+ * resources - the memento pattern operates on a single object.
+ * <p>
+ * In this example the object ({@link Star})
  * gives out a "memento" ({@link StarMemento}) that contains the state of the object.
  * Later on the memento can be set back to the object restoring the state.
  *