[FLINK-5151][Documentation] Add discussion about object mutations to …

[masteryhx]

What is the purpose of the change

Add discussion about whether it is safe to reuse objects for different state backends to state backend docs.

Brief change log

• Add discussion in The HashMapStateBackend and The EmbeddedRocksDBStateBackend of state_backends.md.

Verifying this change

This change is trivial without any test coverage.

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

[flinkbot]

CI report:

• abf1ab6 Azure: SUCCESS

Bot commands

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

[Myasuka]

Thanks for creating the PR. I think we also need to update related chinese documentation.

[masteryhx]

Thanks for creating the PR. I think we also need to update related chinese documentation.

Thanks for the remind. I have also updated the related chinese documentation.

[Myasuka]

Thanks for the update, LGTM.