docs(guides): mention limitation of Worker api (webpack#6436)

Author: chenxsan

src/content/guides/web-workers.mdx

@@ -15,6 +15,8 @@ new Worker(new URL('./worker.js', import.meta.url));
 
 The syntax was chosen to allow running code without bundler, it is also available in native ECMAScript modules in the browser.
 
+Note that while the [`Worker` API](https://developer.mozilla.org/en-US/docs/Web/API/Worker/Worker) suggests that `Worker` constructor would accept a string representing the URL of the script, in webpack 5 you can only use `URL` instead.
+
 ## Example
 
 **src/index.js**