Summary

Add @since tag for the field JapaneseEra.REIWA

Problem

Without a @since tag, users would think JapaneseEra.REIWA field had been declared public since the class' inception (JDK8).

Solution

Explicitly mention that the field is public since JDK13 with a @since tag.

Specification

Change the field description of java.time.chrono.JapaneseEra.REIWA from:

/**
 * The singleton instance for the 'Reiwa' era (2019-05-01 - )
 * which has the value 3. The end date of this era is not specified, unless
 * the Japanese Government defines it.
 */

to:

/**
 * The singleton instance for the 'Reiwa' era (2019-05-01 - )
 * which has the value 3. The end date of this era is not specified, unless
 * the Japanese Government defines it.
 *
 * @since 13
 */