ICU-12973 CLDR Japanese Era data and tentative Japanese new era support

icu4c/source/i18n/unicode/calendar.h

@@ -167,6 +167,19 @@ class BasicTimeZone;
  * to ~5,800,000 CE. Programmers should use the protected constants in `Calendar` to
  * specify an extremely early or extremely late date.
  *
+ * <p>
+ * The Japanese calendar uses a combination of era name and year number.
+ * When an empror of Japan abdicates and a new emperor ascends the throne,
+ * a new era is declared and year number is reset to 1. Even if the date of
+ * abdication is scheduled ahead of time, the new era name might not be
+ * announced until just before the date. In such case, ICU4C may include
+ * a start date of future era without actual era name, but not enabled
+ * by default. ICU4C users who want to test the behavior of the future era
+ * can enable the tentative era by:
+ * <ul>
+ * <li>Environment variable <code>ICU_ENABLE_TENTATIVE_ERA=true</code>.</li>
+ * </ul>
+ *
  * @stable ICU 2.0
  */
 class U_I18N_API Calendar : public UObject {

icu4c/source/i18n/unicode/ucal.h

@@ -142,7 +142,7 @@
  * <p>
  * The Japanese calendar uses a combination of era name and year number.
  * When an empror of Japan abdicates and a new emperor ascends the throne,
- * a new era is declared and year number is reset to 1. Even the date of
+ * a new era is declared and year number is reset to 1. Even if the date of
  * abdication is scheduled ahead of time, the new era name might not be
  * announced until just before the date. In such case, ICU4C may include
  * a start date of future era without actual era name, but not enabled