Add streaming timeout documentation to three pages:
- guides/local-llm-on-mac.md: New 'Timeouts' section with table of all
three timeouts, their defaults, local auto-adjustments, and env var
overrides
- reference/faq.md: Tip box in the local models FAQ section
- user-guide/configuration.md: 'Streaming Timeouts' subsection under
the agent config section
Follow-up to #6967.