[API Shield] Document default JWK algorithm

[janrueth]

Summary

Documents API Shield JWT validation behavior when an RSA JSON Web Key omits alg:

• Defaults the effective algorithm to RS256 for compatibility.
• Recommends always providing alg explicitly.
• Documents response normalization and informational message 110003.
• Clarifies that the effective algorithm must match the JWT header.

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Documentation checklist

• The change adheres to the documentation style guide.

[cloudflare-docs-bot]

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✅ No issues found in commit ec3f8cc.

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