RFR: JDK-8072456 @since tags missing from TimeUnit

[Staffan Larsen]

The MINUTES, HOURS, and DAYS fields of TimeUnit are missing an "@since 1.6" tag.

webrev: http://cr.openjdk.java.net/~sla/8072456/webrev.00/ <http://cr.openjdk.java.net/~sla/8072456/webrev.00/>
bug: https://bugs.openjdk.java.net/browse/JDK-8072456 <https://bugs.openjdk.java.net/browse/JDK-8072456>

Thanks,
/Staffan