docs(apikit-asyncapi): clarify that XML validation is not supported

[stn1slv]

Add an explicit NOTE at the start of the "Validation of Messages" section stating that XML message validation is not supported and that only JSON payloads are validated against schemas.

Rationale:

• Prevent confusion for users expecting XML schema validation
• Align parameter wording with actual validation capabilities

This product limitation has been confirmed in GUS ID W-16756836

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[Copilot]

Pull Request Overview

Adds clarification that XML message validation is not supported in the APIkit AsyncAPI module documentation. This addresses potential user confusion by explicitly stating that only JSON payloads can be validated against schemas.

Key changes:

• Added a NOTE block at the beginning of the "Validation of Messages" section
• Explicitly states XML validation limitation to align documentation with actual product capabilities

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[stn1slv]

This product limitation has been confirmed in GUS ID W-16756836

[stn1slv]

@Crispy-Salesforce, could you please provide a status update?