Remove Negative Values in changefeed.sink_io_workers

[hegdeadithyak]

This PR updates the description of changefeed.sink_io_workers to match the actual behavior.
Any value <= 0 (including negative values and 0) now clearly means a default worker count will be chosen automatically, while values > 0 use the given number.

Fixes: #156774

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[aerfrei]

This is a good catch and worth merging. Please squash down to one commit and make sure it has a commit message that conforms to the repo's policy.

[aerfrei]

Please squash and edit the commit message.

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

Thank you for updating your pull request.

Before a member of our team reviews your PR, I have some potential action items for you:

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Your pull request contains more than 1000 changes. It is strongly encouraged to split big PRs into smaller chunks.

It looks like your PR touches SQL parser code but doesn't add or edit parser tests. Please make sure you add or edit parser tests if you edit the parser.

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[hegdeadithyak]

Apologies everyone — I believe I made a mistake while squashing the commits. I’ve corrected it now, but I’m not sure why it’s still requesting a review.