RFR: 8374905: Clarify ZonedDateTime#toString() documentation regarding omitted zero seconds

[Naoto Sato]

Clarifying the javadoc for `ZonedDateTime#toString()` about the zero seconds/nanoseconds omission. A corresponding CSR has also been drafted.

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/29146/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29146&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374905
  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/29146.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29146/head:pull/29146

PR: https://git.openjdk.org/jdk/pull/29146