[FLINK-31868] Fix DefaultInputSplitAssigner javadoc for class

[pvary]

What is the purpose of the change

Fix the discrepancy between the code and the comment. The comment erroneously states that the return order for the splits is in order.

Brief change log

The decision was to update the comment

Verifying this change

No testing is needed

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? JavaDocs

[flinkbot]

CI report:

• 21efa11 Azure: SUCCESS

Bot commands

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• @flinkbot run azure re-run the last Azure build

[zhuzhurk]

Thanks for creating this PR! @pvary
LGTM.

[pvary]

Thanks @zhuzhurk for the guidance and commit!