Follow-up to #29042 (xAI Web Search provider plugin). Adds xAI to the
canonical user-facing and developer-facing docs, with the search-only
caveat and the LLM-in-a-trench-coat trust model carried over from the
class docstring.
- user-guide/features/web-search.md
- Backends table: new xAI row + extended search-only note
- New 'xAI (Grok)' setup section with config knobs and trust-model
caution admonition
- Single-backend yaml comment now lists 'xai'
- Auto-detection table: explicitly note that xAI is NOT auto-detected
(XAI_API_KEY is shared with inference/TTS/image-gen so we don't
silently take over web for users who only set it for chat)
- developer-guide/web-search-provider-plugin.md
- Added plugins/web/xai/ to the 'study these next' reference list
- reference/environment-variables.md
- XAI_API_KEY description now also mentions web search
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