Add cascade-delete finalizer to stateless apps; document the distinction

apps/metallb.yaml

@@ -18,6 +18,13 @@ kind: Application
 metadata:
   name: metallb
   namespace: argocd
+  # Without this finalizer, deleting this Application object only makes
+  # Argo CD forget about its resources — MetalLB's Pods, CRDs, and our
+  # BGPPeer/BGPAdvertisement would keep running, orphaned. This finalizer
+  # makes deletion cascade properly. Safe here because MetalLB holds no
+  # data of its own — contrast with apps/longhorn.yaml below.
+  finalizers:
+    - resources-finalizer.argocd.argoproj.io
 spec:
   project: default