RFR 9: 8142539 : Incorrect definition of ZoneOffset.MIN and other javadoc cleanup

[Roger Riggs]

Please review a trio of javadoc fixes for java.time.

Webrev:
   http://cr.openjdk.java.net/~rriggs/webrev-typos-8142539/
Issues:
   8142539: Incorrect definition of ZoneOffset.MIN
   8140211: Example in the Documentation is wrong for 
java.time.ZonedDateTime.minusHours
   8139529: java.time.temporal.ChronoUnit.FOREVER typo

Thanks, Roger