docs: add note explaining why x-fern-default is used instead of plain default

[Swimburger]

Summary

Adds a <Note> callout to the default values extension page explaining why Fern uses x-fern-default instead of the standard OpenAPI default property.

The key distinction: OpenAPI default describes the value the server assumes when a parameter is omitted (a documentation hint), while x-fern-default instructs the SDK to actively send the value in every request, ensuring it's always present on the wire.

Review & Testing Checklist for Human

• Verify the note renders correctly on the preview deployment
• Confirm the semantic explanation is technically accurate

Notes

Low-risk, single-callout addition to an existing page.

Link to Devin session: https://app.devin.ai/sessions/8dbd9e9711624a8d8a8ef82cfcdd0134
Requested by: @Swimburger

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[github-actions]

🌿 Preview your docs: https://fern-preview-8c3bb049-6d21-480f-a029-c11c6613c9c3.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/api-definitions/openapi/extensions/default-values