Define Japanese new Era character

Summary

This is a follow-on CSR to JDK-8211739. Add a paragraph for that behavioral change.

Problem

The existing JCK test for JDK12 fails on that code point.

Solution

Add a paragraph in java.lang.Character class description explaining the change.

Specification

Insert the following paragraph at the end of the first part of the java.lang.Character class description, before the "Unicode Character Representation" heading:

 * The code point, U+32FF, is reserved by the Unicode Consortium
 * to represent the Japanese square character for the new era that begins
 * from May, 2019. Relevant methods in the Character class return the same
 * properties as for the existing Japanese era characters (e.g., U+337E for
 * "Meizi"). For the details of the code point, refer to
 * http://blog.unicode.org/2018/09/new-japanese-era.html