[FLINK-39652][docs] Improve documentation for regexp functions

[raminqaf]

What is the purpose of the change

Improves and updates the docs of regexp functions.

Brief change log

• updated the sql_functions file to mirror the correct behavior of the regexp function family.

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: (yes / no / don't know)
• 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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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: Claude Opus 4.8

[flinkbot]

CI report:

• 86e2515 UNKNOWN

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[fhueske]

Thanks for the docs fix @raminqaf!

I'll merge this PR.