mention polyfill in migration guide

documentation-v3/src/pages/upgrade-v3/index.md

@@ -3,7 +3,7 @@ layout: "@layouts/DocLayout.astro"
 title: "Upgrade to Lucia v3"
 ---
 
-Version 3.0 rethinks Lucia and the role it should play into your application. We have stripped out all the annoying bits, and everything else we kept has been refined even more. Everything is more flexible, and just all around easier to understand and work with. 
+Version 3.0 rethinks Lucia and the role it should play into your application. We have stripped out all the annoying bits, and everything else we kept has been refined even more. Everything is more flexible, and just all around easier to understand and work with.
 
 We estimate it shouldn't take longer than an hour to upgrade your project. If you're having issues with the migration or have any questions, feel free to ask on our [Discord server](https://discord.com/invite/PwrK3kpVR3). Click [here for the full changelog]().
 
@@ -103,6 +103,16 @@ declare module "lucia" {
 }
 ```
 
+### Polyfill
+
+`lucia/polyfill/node` has been removed. Manually polyfill the Web Crypto API by importing the `crypto` module.
+
+```ts
+import { webcrypto } from "node:crypto";
+
+globalThis.crypto = webcrypto as Crypto;
+```
+
 ## Update your database
 
 Refer to each database migration guide: