[FLINK-23582][docs][table] Add documentation for session window tvf.

[beyond1920]

What is the purpose of the change

This pr adds documentation for session window tvf.

Brief change log

• Window-tvf.md
• window-agg.md

Verifying this change

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? (docs)

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Last check on commit fd63386 (Tue Aug 24 10:52:32 UTC 2021)

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[godfreyhe]

Thanks for the contribution, @beyond1920. I left some minor comments

[beyond1920]

@godfreyhe Thanks for review, I've updated. Please have a look.

[godfreyhe]

cc @wuchong

[wuchong]

The documentation looks good to me. However, it seems that the session window TVF syntax missed to declare partition key. This introduces a wrong sematic when assiging session windows. We should fix it ASAP.

[beyond1920]

@wuchong wu, Thanks a lot for pointing this, I've submit a pr to fix it.

[wuchong]

Could you update the example to show a partitioned session window? I think that is a more usable use case.

[beyond1920]

done

[beyond1920]

@wuchong Jark, I've updated the pr to add an example to show a partitioned session window and rebase, please have a look again.

[tillrohrmann]

@wuchong can we merge this PR?

[wuchong]

@tillrohrmann , we discussed to rework the syntax of session window TVF in FLINK-24024. There would be a new docs PR to illustrate the new syntax. So I think we can close this PR for now. What do you think @beyond1920 ?

[beyond1920]

@wuchong @tillrohrmann I prefer to close this pull request. I would create a new pull request after we upgrade the syntax of session window.