Added Javadoc to exDate field in AccountTransaction

Issue: portfolio-performance#5439

Author: buchen

name.abuchen.portfolio/src/name/abuchen/portfolio/model/AccountTransaction.java

@@ -47,6 +47,18 @@ public String toString()
 
     private Type type;
 
+    /**
+     * Ex-date of this transaction.
+     * <p>
+     * For dividend payments, this is the ex-dividend date: the first trading
+     * day on which the security is traded without entitlement to the declared
+     * dividend. Only shareholders who own the security before this date receive
+     * the dividend.
+     * <p>
+     * The ex-date may also be used for other transaction types to map them to
+     * the period a security was held (for example, taxation of retained income
+     * booked at a later time).
+     */
     private LocalDateTime exDate;
 
     public AccountTransaction()