[FLINK-34147][table-planner] Enhance the java doc of TimestampData to distinguish the different usage of Instant and LocalDateTime

[swuferhong]

What is the purpose of the change

Currently, the java docs of TimestampData is ambiguous. TimestampData represents both Instant and LocalDateTime. this class use fromInstant()/toInstant() to convert an Instant from/to TimestampData, and use fromLocalDateTime()/toLocalDateTime() to convert a LocalDateTime from/to TimestampData. This need to be indicated in java docs.

Brief change log

Verifying this change

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? no docs

[flinkbot]

CI report:

• 1e0acf858f5dfa9a029dfc0507d345cdd36a17b3 Azure: SUCCESS

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[swuferhong]

Hi, @lirui Could you help review this, please?