Docs xmlnode deprecations followup

[anukalp2804]

This pull request completes the deprecation documentation for XmlNode
constants and methods by adding consistent Javadoc rationale and
explicit replacement references.

No functional behavior is changed.

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[anukalp2804]

Hi @desruisseaux , kindly requesting a review when time permits.
This PR is documentation-only and clarifies deprecation rationale for XmlNode APIs.
Please also approve the workflow run if appropriate. Thank you!