Enhance javadoc.

library/src/main/java/org/mustangproject/SubjectCode.java

@@ -2,7 +2,25 @@ package org.mustangproject;
 
 /**
  * EN16931-ID: BT-21 - the qualification of the free text on the invoice from BT-22
- * In the first step only the recommended codes are implemented. 
+ * @see <a href="https://service.unece.org/trade/untdid/d96a/uncl/uncl4451.htm">UNTDID - D.96A - Element 4451</a> for a complete list of codes
+ *
+ * In the first step only the recommended codes are implemented:
+ * AAI : General information / Allgemeine Informationen
+ * SUR : Supplier remarks / Anmerkungen des Verkäufers
+ * REG : Regulatory information / Regulatorische Informationen
+ * ABL : Government information / Rechtliche Informationen
+ * TXD : Tax declaration / Informationen zur Steuer
+ * CUS : Customs declaration information / Zollinformationen
+ *
+ * plus
+ * ACY : Introduction
+ * AAK : Price conditions
+ * ABZ : Instructions/information about revolving documentary credit
+ * PMT : Payment information
+ * PMD : Payment detail/remittance information
+ * AAB : Terms of payments
+ * ACB : Additional information
+ * INV : Invoice instruction
  */
 public enum SubjectCode {
   /**
@@ -38,29 +56,28 @@ public enum SubjectCode {
    */
   AAK,
   /**
-   * Vehicle licence number
+   * Instructions/information about revolving documentary credit
    */
   ABZ,
-	/**
-	* Payment information
-	*/
-	PMT,
-	/**
-	 * Payment detail/remittance information
-	 */
-	
-	PMD, 
-	/**
-	 * Payment term
-	 */
-	AAB,
-	/**
-	 * Additional information
-	 */
-	ACB,
-	/**
-	 * Invoice instruction
-	 */
-	INV
+  /**
+   * Payment information
+   */
+  PMT,
+  /**
+   * Payment detail/remittance information
+   */
+  PMD,
+  /**
+   * Payment term
+   */
+  AAB,
+  /**
+   * Additional information
+   */
+  ACB,
+  /**
+   * Invoice instruction
+   */
+  INV
 
 }