Update usage-considerations.mdx

added info on what happens if Workers AI is down

Author: kathayl

src/content/docs/ai-gateway/guardrails/usage-considerations.mdx

@@ -11,6 +11,7 @@ Since Guardrails runs on Workers AI, enabling it incurs usage on Workers AI. You
 
 ## Additional considerations
 
+- Model availability: If at least one hazard category is set to `block`, but AI Gateway is unable to receive a response from Workers AI, the request will be blocked.
 - Latency impact: Enabling Guardrails adds some latency. Consider this when balancing safety and speed.
 
 :::note