Add documentation for new --application-namespace option

Signed-off-by: Graham Beckley <gbeckley@mozilla.com>

Author: grahamalama

docs/install/reference.md

@@ -26,6 +26,10 @@ Runs the Argo CD Image Updater, possibly in an endless loop.
 
 ### Flags
 
+**--application-namespace *namespace* **
+
+Specifies the Kubernetes namespace in which Argo Image Updater will manage Argo CD Applications when using the Kubernetes-based Application API. By default, applications in all namespaces are considered. This flag can be used to limit scope to a single namespace for performance, security, or organizational reasons.
+
 **--argocd-auth-token *token* **
 
 Use *token* for authenticating to the Argo CD API. This token must be a base64