update: add CyberArk REST API details

This should be documented since the CyberArk REST API returns a JSON payload with `UserName` for username and `Content` for password.

See code here for mapping:
https://github.com/newrelic/infrastructure-agent/blob/master/pkg/databind/internal/secrets/cyberarkapi.go#L45-L50

src/content/docs/infrastructure/host-integrations/installation/secrets-management.mdx

@@ -613,6 +613,14 @@ variables:
           insecure_skip_verify: true
 ```
 
+<Callout variant="important">
+  The the above example, you'll need to call the username and password like this:
+  ```
+   USERNAME: ${credentials.user}
+   PASSWORD: ${credentials.password}
+ ```
+</Callout>
+
 ## New Relic CLI Obfuscation [#newrelic-cli-obfuscation]
 
 <Callout variant="important">