Document backend capacity management

engine.go

@@ -65,7 +65,7 @@ var (
 	ErrPromptRender = errors.New("failed to render prompt")
 	// ErrCapacityExceeded identifies a Run rejected because the selected backend
 	// already admitted ConcurrencyLimit + QueueCapacity calls. It is not an
-	// invalid request, provider rate limit, or ErrLLMGenerate.
+	// invalid request, an LLM or provider rate-limit response, or ErrLLMGenerate.
 	ErrCapacityExceeded = errors.New("backend capacity exceeded")
 	// ErrLLMGenerate identifies a model-client failure or a nil successful
 	// response. Errors returned by an injected LLMClient remain available