docs(jwk): clarify what the JWK middleware validates

[surajkewat72]

Updated the documentation to explain what validations happen automatically vs what developers need to handle themselves. Added some examples showing different use cases including custom claim validation.

Fixes #4542

The author should do the following, if applicable

• [ ] Add tests
• [ ] Run tests
• [ ] bun run format:fix && bun run lint:fix to format the code
• [ ] Add TSDoc/JSDoc to document the code

[yusukebe]

@surajkewat72

Thank you for the PR. I think we have to prioritize updating the website: https://hono.dev/docs/middleware/builtin/jwk , instead of adding a lot of description to JSDoc.