[FLINK-9790] [doc] Add documentation for UDF in SQL Client

[xccui]

What is the purpose of the change

This PR aims to add document for UDF in SQL Client.

Brief change log

• Add related document for SQL Client UDF in sqlClient.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, Yarn/Mesos, 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)

[twalthr]

Thank you @xccui. The documentation looks very good. I will take care of merging this.

[xccui]

Thanks for the review, @twalthr.