Summary
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Clarify the language used for the public singleton definition in JapaneseEra class.

Problem
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With the change to 8212941, the class is now proofed for future JapaneseEra additions without spec changes. However, it is not clear whether the "subsequent versions of this class" refers to a feature release or an update release.

Solution
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Make it explicit that the singleton additions are intended for future platform releases.

Specification
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Change the ambiguous sentence in the class description of j.t.c.JapaneseEra as follows:

    @@ -94,7 +94,7 @@
      * dates before Meiji 6, January 1 are not supported.
      * The number of the valid eras may increase, as new eras may be
      * defined by the Japanese government. Once an era is defined,
    - * subsequent versions of this class will add a singleton instance
    + * future versions of the platform may add a singleton instance
      * for it. The defined era is expected to have a consecutive integer
      * associated with it.
      *