docs: add top level await pitfall

docs/pitfalls.md

@@ -33,3 +33,11 @@ Although the explicit error may annoy developer at first, it does save your time
 
 You can try to use the methods mentioned in the official FAQ to resolve the issue, or you can try the [`@document-kits/viewer`](https://priestch.github.io/document-viewer), it has
 a `disableCORSCheck` option to disable the CORS check for quick start. Be careful, you should not enable this option in production if you don't know what it means.
+
+### Top-level await is not available in the configured target environment
+
+The reason for this error is that the source code of `pdf.js` contains `top-level await` statement, when used and bundled
+in a project, it may cause the error. As I commented in the issue https://github.com/mozilla/pdf.js/issues/17245#issuecomment-1918453195,
+it can be resolved add/modify related gulp task.
+
+A good article about `top-level await` is [here](https://github.com/orgs/web-infra-dev/discussions/9).