Release Note: Improvements in Serial GC Young pause time report

With Serial GC, when Young GC fails and upgrades to Full GC, the logging messages may be confusing. In previous releases, the log would say both "Pause Full" and "Pause Young" finished at the same time, leading to double-counting the pause time. In this release, the logging is fixed to properly reflect times spent in "Young" and "Full" collections. See more at JDK-8215221.