updated the post-render documentation on action.Upgrade

Signed-off-by: Matt Butcher <matt.butcher@microsoft.com>

pkg/action/upgrade.go

@@ -87,6 +87,10 @@ type Upgrade struct {
 	SubNotes bool
 	// Description is the description of this operation
 	Description string
+	// PostRender is an optional post-renderer
+	//
+	// If this is non-nil, then after templates are rendered, they will be sent to the
+	// post renderer before sending to the Kuberntes API server.
 	PostRenderer postrender.PostRenderer
 	// DisableOpenAPIValidation controls whether OpenAPI validation is enforced.
 	DisableOpenAPIValidation bool