Clarify transaction boundary recommendation

[pch8388]

This PR clarifies the intent of the documentation regarding the usage of @Transactional in the repository layer.

It adds a note to explain that while the example shows @Transactional on repository methods for read-only defaults, it is generally recommended to define transaction boundaries at the service layer to reflect business logic and ensure proper control.

Fixes #3842

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[mp911de]

Thank you for your contribution. That's merged, polished, and backported now.

[mp911de]

Please mind to DCO-sign your commits for future pull requests.

[pch8388]

Thank you for reviewing and merging the PR!
I’m glad I could help improve the documentation even a little.
Looking forward to contributing again in the future.