docs: document API key enablement, server-side limits, cleanup and processing status on the knowledge base sync page

[silentoplayz]

Summary

I compared the knowledge base sync page with the knowledge and files routers and the API key checks.

• The endpoints are named in full (POST /api/v1/knowledge/{id}/sync/diff and .../sync/cleanup), and the manifest entry carries a size field as well.
• Cleanup deletes a stale file's record and bytes. It keeps them when ENABLE_KNOWLEDGE_FILE_RETENTION is on, when the file is linked elsewhere, or when the caller is neither owner nor admin. A file with no stored hash is re-uploaded once on the first sync.
• The server enforces its own upload limits (RAG_FILE_MAX_SIZE answered with HTTP 413, RAG_ALLOWED_FILE_EXTENSIONS with HTTP 400) regardless of oikb's max-size.
• Processing runs in the background and GET /api/v1/files/{id}/process/status reports completion.
• API keys are off by default and gated by a permission. An HTTP 403 has three causes, including endpoint restrictions that must allow /api/v1/knowledge and /api/v1/files.

Related issue or discussion

None.

Checklist

• I have reviewed the relevant documentation and matched the existing style.
• This PR meets Open WebUI's contribution standards: it is accurate, relevant to users, narrowly scoped, maintainable, and not promotional content, advertising, lead generation, SEO placement, or a request to list a product, service, provider, integration, gateway, tool, or company primarily for visibility.
• I understand that PRs that do not meet these standards may be closed without review and will not be merged. Repeated, low-quality, off-topic, promotional, or intentionally misleading submissions may result in the contributor being blocked from future participation in Open WebUI repositories.

Notes for reviewers

Sources on dev: routers/knowledge.py (FileManifestEntry, the sync routes, cleanup), routers/files.py (size and extension checks, process/status), config.py (ENABLE_API_KEYS, ENABLE_API_KEYS_ENDPOINT_RESTRICTIONS, API_KEYS_ALLOWED_ENDPOINTS), and utils/auth.py. oikb's own flags are unchanged.