Add javadoc

Author: leonardehrenfried

src/main/java/org/rutebanken/time/XmlDateTime.java

@@ -23,6 +23,10 @@ public XmlDateTime(LocalDateTime localDateTime) {
     this.localDateTime = Objects.requireNonNull(localDateTime);
   }
 
+  /**
+   * Converts the abstract XML dateTime into a {@link ZonedDateTime}. If the underlying date time
+   * was already zoned, then it is converted to the same instant but in the specified zoned.
+   */
   public ZonedDateTime atZone(ZoneId zoneId) {
     return zonedDateTime != null ? zonedDateTime.withZoneSameInstant(zoneId) : localDateTime.atZone(zoneId);
   }