docs: fix kustomize patches example

Related to: argoproj#16352 Also, add an ApplicationSet example. 

Signed-off-by: Nicholas Morey <nicholas@morey.tech>

docs/user-guide/kustomize.md

@@ -21,18 +21,17 @@ To use Kustomize with an overlay, point your path to the overlay.
     If you're generating resources, you should read up how to ignore those generated resources using the [`IgnoreExtraneous` compare option](compare-options.md).
 
 ## Patches
-Patches are a way to kustomize resources using inline configurations in Argo CD applications.  This allows for kustomizing without  kustomization file.  `patches`  follow the same logic as the corresponding Kustomization.  Any patches that target existing Kustomization file will be merged.
+Patches are a way to kustomize resources using inline configurations in Argo CD applications.  `patches`  follow the same logic as the corresponding Kustomization.  Any patches that target existing Kustomization file will be merged.
 
-The following Kustomization can be done similarly in an Argo CD application.
+This Kustomizate example sources manifests from the `/kustomize-guestbook` folder of the `argoproj/argocd-example-apps` repository and patches the `Deployment` to use port `443` on the container.
 ```yaml
 apiVersion: kustomize.config.k8s.io/v1beta1
 kind: Kustomization
 metadata:
   name: kustomize-inline-example
 namespace: test1
 resources:
-  - https://raw.githubusercontent.com/argoproj/argocd-example-apps/master/guestbook/guestbook-ui-deployment.yaml
-  - https://raw.githubusercontent.com/argoproj/argocd-example-apps/master/guestbook/guestbook-ui-svc.yaml
+  - https://raw.githubusercontent.com/argoproj/argocd-example-apps/master/kustomize-guestbook/
 patches:
   - target:
       kind: Deployment
@@ -42,7 +41,8 @@ patches:
         path: /spec/template/spec/containers/0/ports/0/containerPort
         value: 443
 ```
-Application will clone the repository, use the specified path, then kustomize using inline patches configuration.
+
+This `Application` does the equivalent using the inline `kustomize.patches` configuration.
 ```yaml
 apiVersion: argoproj.io/v1alpha1
 kind: Application
@@ -57,7 +57,7 @@ spec:
     server: https://kubernetes.default.svc
   project: default
   source:
-    path: guestbook
+    path: kustomize-guestbook
     repoURL: https://github.com/argoproj/argocd-example-apps.git
     targetRevision: master
     kustomize:
@@ -71,6 +71,45 @@ spec:
               value: 443
 ```
 
+The inline kustomize patches work well with `ApplicationSets`, too. Instead of maintaining a patch or overlay for each cluster, patches can now be done in the `Application` template and utilize attributes from the generators. For example, with [`external-dns`](https://github.com/kubernetes-sigs/external-dns/) to set the [`txt-owner-id`](https://github.com/kubernetes-sigs/external-dns/blob/e1adc9079b12774cccac051966b2c6a3f18f7872/docs/registry/registry.md?plain=1#L6) to the cluster name.
+
+```yaml
+apiVersion: argoproj.io/v1alpha1
+kind: ApplicationSet
+metadata:
+  name: external-dns
+spec:
+  goTemplate: true
+  goTemplateOptions: ["missingkey=error"]
+  generators:
+  - clusters: {}
+  template:
+    metadata:
+      name: 'external-dns'
+    spec:
+      project: default
+      source:
+        repoURL: https://github.com/kubernetes-sigs/external-dns/
+        targetRevision: v0.14.0
+        path: kustomize
+        kustomize:
+          patches:
+          - target:
+              kind: Deployment
+              name: external-dns
+            patch: |-
+              - op: add
+                path: /spec/template/spec/containers/0/args/3
+                value: --txt-owner-id={{.name}}   # patch using attribute from generator
+      destination:
+        name: 'in-cluster'
+        namespace: default
+      syncPolicy:
+        automated:
+          prune: true
+          selfHeal: true
+```
+
 ## Private Remote Bases
 
 If you have remote bases that are either (a) HTTPS and need username/password (b) SSH and need SSH private key, then they'll inherit that from the app's repo.