[FLINK-37797] Model documentation

[lihaosky]

What is the purpose of the change

Documentation for model DDL

Verifying this change

Built doc locally

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• 422ffe6 Azure: SUCCESS

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[fsk119]

Thanks for your contribution. I left some comments.

[fsk119]

It's better use

SELECT * FROM
ML_PREDICT(
TABLE input_table,
  MODEL model_name,
  DESCRIPTOR(feature_columns),
[MAP['key', 'value']]

Because flink only supports:

1. all arguments use named argument
2. all arguments don't use named argument

We get an error for the following statement:

SELECT id, len, z
FROM ML_PREDICT(TABLE nullable_src, MODEL m2, DESCRIPTOR(`id`, `len`), CONFIG => MAP['async','false']) 

[fsk119]

LGTM