Clarify Model resource concept description

Refine the Model section in docs/concepts.md to directly describe it as a
declarative configuration resource for model providers, parameters, and
credentials rather than using colloquial phrasing.

Fixes #356

Author: rakyll

docs/concepts.md

@@ -33,6 +33,6 @@ A binding can also carry a `goal`, a plain-language description of the environme
 
 ## Model
 
-A `Model` is not a model. It is a named model configuration: which provider to call, which model identifier to use, provider-specific generation parameters such as temperature, and a reference to the Kubernetes secret holding the API key.
+A configuration for a model provider and its parameters: which provider to call, the model identifier, provider-specific generation parameters such as max_tokens, and references to the Kubernetes secret holding the credentials.
 
-Declaring it as a resource is what makes it manageable across the cluster. The configuration lives in one place instead of in every agent's environment, so rotating a key, pinning a new model version, or tightening a parameter is one `ax apply` rather than a hunt through task definitions. AX's own components read it too, for example when planning a workspace from a goal.
+Declaring models as cluster resources makes them manageable and reusable across tasks. The configuration lives in one place instead of in every agent's environment, so rotating a key, pinning a new model version, or tuning parameters is a single `ax apply` rather than a hunt through task definitions. AX's own components read `Model` resources as well, for example when planning a workspace from a goal.