Update policy-service.mdx

Signed-off-by: Eric Gregory <[email protected]>

docs/deployment/security/policy-service.mdx

@@ -23,7 +23,7 @@ The policy service evaluates the provided action and returns a response indicati
 
 ### Policy Requests and Responses
 
-Policy requests are made on a NATS subject, which is specified by the `--policy-topic` [host config](/docs/reference/host-config) option.
+Policy requests are made on a NATS subject, which is specified by the `--policy-topic` [host config](/docs/reference/host-config) option. (The argument uses the general messaging term "topic" rather than the NATS-specific "subject," but you will simply specify a [NATS subject](/docs/ecosystem/nats/#essential-nats-concepts) here.)
 
 <details>
 <summary>Policy request schemas</summary>