docs: clarify codex max defaults and xhigh availability (#7449)

## 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.

docs/config.md

@@ -195,6 +195,7 @@ If the selected model is known to support reasoning (for example: `o3`, `o4-mini
 - `"low"`
 - `"medium"` (default)
 - `"high"`
+- `"xhigh"` (available only on `gpt-5.1-codex-max`)
 
 Note: to minimize reasoning, choose `"minimal"`.