Document restarting argocd after modifying argocd-cm (argoproj#12405)

Signed-off-by: Lie Ryan <lie.1296@gmail.com>
Co-authored-by: Blake Pettersson <blake.pettersson@gmail.com>
Signed-off-by: Kevin Lyda <kevin@lyda.ie>

docs/user-guide/kustomize.md

@@ -131,6 +131,9 @@ data:
     kustomize.buildOptions: --load-restrictor LoadRestrictionsNone
     kustomize.buildOptions.v4.4.0: --output /tmp
 ```
+
+After modifying `kustomize.buildOptions`, you may need to restart ArgoCD for the changes to take effect.
+
 ## Custom Kustomize versions
 
 Argo CD supports using multiple Kustomize versions simultaneously and specifies required version per application.