capture cancellation reason/comment, add callback docs

example/convex/http.ts

@@ -6,6 +6,26 @@ const http = httpRouter();
 polar.registerRoutes(http, {
   // Optional custom path, default is "/events/polar"
   path: "/events/polar",
+  // Optional callback for when a subscription is updated
+  onSubscriptionUpdated: async (ctx, event) => {
+    console.log("Subscription updated", event);
+    if (event.data.customerCancellationReason) {
+      console.log(
+        "Customer cancellation reason",
+        event.data.customerCancellationReason
+      );
+      console.log(
+        "Customer cancellation comment",
+        event.data.customerCancellationComment
+      );
+    }
+    // This callback is run in an Action, so you could pipe this customer
+    // cancellation reason to another service, for example.
+  },
+  // Other available callbacks:
+  onSubscriptionCreated: undefined,
+  onProductCreated: undefined,
+  onProductUpdated: undefined,
 });
 
 export default http;