Users were setting model.provider to "main" after reading the auxiliary
provider docs, causing "Unknown provider" errors. The "main" alias is
only valid inside auxiliary:, compression:, and fallback_model: configs
where it means "use the same provider as my main agent chat."
Added warning admonitions and inline clarifications to:
- configuration.md: Auxiliary Models provider list and Provider Options table
- fallback-providers.md: Provider Options for Auxiliary Tasks table
Reported by community member cn on Discord.