docs: document RAG file context scope

[Eole7]

Add docs for open-webui/open-webui#26151

[Copilot]

Pull request overview

Adds documentation for the new RAG “File Context Scope” behavior so admins and plugin authors understand where attached-file context is injected (conversation-level vs message-level), and how it interacts with RAG_SYSTEM_CONTEXT.

Changes:

• Documented the new RAG_FILE_CONTEXT_SCOPE environment variable and clarified interaction with RAG_SYSTEM_CONTEXT.
• Added user-facing docs describing the “File Context Scope” admin setting and when to use each mode.
• Updated plugin filter docs to reflect the new injection options for attached-file context.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
docs/troubleshooting/rag.mdx	Adds a troubleshooting note pointing readers to File Context Scope / RAG_FILE_CONTEXT_SCOPE.
docs/reference/env-configuration.mdx	Documents RAG_FILE_CONTEXT_SCOPE and updates RAG_SYSTEM_CONTEXT description to reference it.
docs/features/extensibility/plugin/functions/filter.mdx	Updates the built-in RAG pipeline description to mention message-scoped file context.
docs/features/chat-conversations/rag/index.md	Adds a new “File Context Scope” section explaining both modes and precedence with RAG_SYSTEM_CONTEXT.

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