[ACR] az acr manifest list-metadata: Update help messages for untagged manifests (#30382)

Author: bbishopMSFT

src/azure-cli/azure/cli/command_modules/acr/_help.py

@@ -704,6 +704,10 @@
     text: az acr manifest list-metadata -r myregistry -n hello-world
   - name: List the metadata of the manifests in the repository 'hello-world'.
     text: az acr manifest list-metadata myregistry.azurecr.io/hello-world
+  - name: List the metadata of all manifests (tagged/untagged) in the repository 'hello-world'.
+    text: az acr manifest list-metadata -r myregistry -n hello-world --query "[]"
+  - name: List the metadata of untagged manifests in the repository 'hello-world'.
+    text: az acr manifest list-metadata -r myregistry -n hello-world --query "[?tags==null]"
 """
 
 helps['acr manifest update-metadata'] = """