HexDump.writeHex is similar to Long#toHexString. Consider consolidating.

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1753050 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/poi/util/HexDump.java

@@ -391,6 +391,10 @@ public class HexDump {
         return sb.toString();
     }
     
+    /**
+     * @see Integer#toHexString(int)
+     * @see Long#toHexString(long)
+     */
     private static void writeHex(StringBuilder sb, long value, int nDigits, String prefix) {
         sb.append(prefix);
         char[] buf = new char[nDigits];