docs: update config docstrings

Author: psychedelicious

invokeai/app/services/config/config_default.py

@@ -108,6 +108,7 @@ class InvokeAIAppConfig(BaseSettings):
         remote_api_tokens: List of regular expression and token pairs used when downloading models from URLs. The download URL is tested against the regex, and if it matches, the token is provided in as a Bearer token.
         scan_models_on_startup: Scan the models directory on startup, registering orphaned models. This is typically only used in conjunction with `use_memory_db` for testing purposes.
         unsafe_disable_picklescan: UNSAFE. Disable the picklescan security check during model installation. Recommended only for development and testing purposes. This will allow arbitrary code execution during model installation, so should never be used in production.
+        allow_unknown_models: Allow installation of models that we are unable to identify. If enabled, models will be marked as `unknown` in the database, and will not have any metadata associated with them. If disabled, unknown models will be rejected during installation.
     """
 
     _root: Optional[Path] = PrivateAttr(default=None)