docs: update auto routing policy to reflect fastest provider selection (#2376)
The `auto` provider selection policy now always selects the fastest
available provider (highest throughput), matching the `:fastest` policy.
This reflects the removal of the author-provider priority shortcut in
the backend (moon-landing#17657).
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>