docs: update cascade (#861)

* docs: update cascade

Signed-off-by: yxxhero <aiopsclub@163.com>

docs/index.md

@@ -217,6 +217,8 @@ helmDefaults:
   reuseValues: false
   # propagate `--post-renderer` to helmv3 template and helm install
   postRenderer: "path/to/postRenderer"
+  #	cascade `--cascade` to helmv3 delete, available values: background, foreground, or orphan, default: background
+  cascade: "background" 
 
 # these labels will be applied to all releases in a Helmfile. Useful in templating if you have a helmfile per environment or customer and don't want to copy the same label to each release
 commonLabels:
@@ -324,6 +326,8 @@ releases:
     skipDeps: false
     # propagate `--post-renderer` to helmv3 template and helm install
     postRenderer: "path/to/postRenderer"
+    # cascade `--cascade` to helmv3 delete, available values: background, foreground, or orphan, default: background
+    cascade: "background" 
 
   # Local chart example
   - name: grafana                            # name of this release