Add value-type notice to java.time classes

The JSR 310 EG is adopting the value-type language defined by the Lambda Expert Group.
This language reinforces the definition of value-types and correct usage with respect to object identity.

"This is a <a href="{@docRoot}/java/lang/doc-files/ValueBased.html">value-based</a>
 class; use of identity-sensitive operations (including reference equality
 ({@code ==}), identity hash code, or synchronization) on instances of
 {@code this class} may have unpredictable results and should be avoided."

The classes with this behavior are:
 - LocalDate, LocalTime, LocalDateTime, ZonedDateTime
 - OffsetTime, OffsetDateTime
 - JapaneseDate, MinguoDate, HijrahDate, ThaiBuddhist
 - Instant, Duration, Period
 - MonthDay, YearMonth, Year
 - ZoneId, ZoneOffset