[FLINK-37148][docs] Update DataStream V2 documents for StateManager#getState and StateManager#getStateOptional methods

[codenohup]

What is the purpose of the change

The PR #26057 adds documentation for DataStream V2.

While writing DataStream V2 examples, we found that the StateManager#getState method was inconvenient for users. To address this, we distinguished between the StateManager#getState and StateManager#getStateOptional methods in #26118.

The documentation also needs to be updated to reflect these changes.

Brief change log

• Update DataStream V2 documents for StateManager#getState and StateManager#getStateOptional methods

This change is a trivial rework / code cleanup 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? not applicable

[flinkbot]

CI report:

• 11a91a7 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

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