docs: clarify MCP searchDocs Ask Fern requirement and result scoping

[devin-ai-integration]

Summary

Two recurring customer questions weren't answered by the existing pages:

• MCP server: searchDocs proxies to Ask Fern, so it fails on every call when Ask Fern is disabled (tool listing still works). Added that, plus the contrast that llms.txt, llms-full.txt, and per-page .md don't require Ask Fern. Also stated explicitly under Authenticated sites that results are always scoped to the caller's roles, so there's no anonymous access to protected content. The mcp: false behavior and the password-to-token exchange were already documented, so they're untouched.
• Ask Fern overview: noted that enabling Ask Fern is a single switch covering the sidebar panel, Ask AI trigger, AI mode in search, and Ask AI buttons on code snippets, and that page-actions options only control page action buttons (no per-surface override).

No new pages; two paragraphs added.

Link to Devin session: https://app.devin.ai/sessions/d4d083e69636452fa7ac72f897359e43

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1785769796-mcp-ask-fern-requirements.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/ai-features/mcp-server
• learn/docs/ai-features/ask-fern/overview