[FLINK-34119][doc] Improve description about changelog in document

[masteryhx]

What is the purpose of the change

Since we have resolved some issues and marked as prodution-ready in release note,

we could update some description about it in doc.

Brief change log

• Remove some descriptions about experimental status
• Update some descriptions about benefits and limitations

Verifying this change

This change is a trivial rework 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

[masteryhx]

@rkhachatryan @fredia @Zakelly Could you help to take a review ? Thanks~

[flinkbot]

CI report:

• d8a1272 Azure: FAILURE

Bot commands

The @flinkbot bot supports the following commands:

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

[fredia]

@masteryhx Thanks for the contribution, LGTM. CI seems broken by https://issues.apache.org/jira/projects/FLINK/issues/FLINK-34077

[masteryhx]

@masteryhx Thanks for the contribution, LGTM. CI seems broken by https://issues.apache.org/jira/projects/FLINK/issues/FLINK-34077

Thanks for the review.
The failed python cases have failed many PR which is not related to this.
So it doesn't block this PR.