RFR: 8276775: ZonedDateTime/OffsetDateTime.toString return invalid ISO-8601 for years <= 1893

[Naoto Sato]

Simple doc clarification where the `toString()` output only conforms to ISO 8601 if the seconds in the offset are zero.

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Commit messages:
 - 8276775: ZonedDateTime/OffsetDateTime.toString return invalid ISO-8601 for years <= 1893

Changes: https://git.openjdk.java.net/jdk/pull/6321/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6321&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276775
  Stats: 7 lines in 2 files changed: 2 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6321.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6321/head:pull/6321

PR: https://git.openjdk.java.net/jdk/pull/6321