Clarify that Request.json raises different error codes:
- 415 Unsupported Media Type: when Content-Type is not application/json
- 400 Bad Request: when body is not valid JSON
This makes the documentation consistent with the API reference
and helps developers understand when each error will occur.
Fixes#5825
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
