docs: CodeChallenge(required=True) only applies to public clients

[ktosiek]

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [x] Other, please describe: documentation change

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I've had to look through code to make sure, but it seems the CodeChallenge is only required when the client uses "none" authentication - which I understand as being a public client. This makes it explicit in the documentation.

• [x] You consent that the copyright of your pull request source code belongs to Authlib's author.

[lepture]

Thanks