Clarify the method description of Duration.toDaysPart()

Summary

Clarify the method description of Duration.toDaysPart()

Problem

The subject method has been added in JDK9, as well as other toXXXPart() methods. However the method description and its behavior is pretty much the same with toDays() method. Users may be confused without clarifying it.

Solution

The method has been added for its consistency with other toXXXPart() methods, so instead of deprecation, adding the explanation where its behavior is the same as toDays() method is appropriate.

Specification

Add the following @apiNote to the method desription of java.time.Duration.toDaysPart() method:

 * @apiNote
 * This method behaves exactly the same way as {@link #toDays()}.