docs: clarify codex max defaults and xhigh availability

[kgruiz]

Summary

Adds the missing xhigh reasoning level everywhere it should have been documented, and makes clear it only works with gpt-5.1-codex-max.

Changes

• docs/config.md

  • Add xhigh to the official list of reasoning levels with a note that xhigh is exclusive to Codex Max.

• docs/example-config.md

  • Update the example comment adding xhigh as a valid option but only for Codex Max.

• docs/faq.md

  • Update the model recommendation to GPT-5.1 Codex Max.
  • Mention that users can choose high or the newly documented xhigh level when using Codex Max.

[etraut-openai]

Thanks for the contribution!