Adds note about public directory (#17203)

This comes up a lot in discussions/as an issue, so added a small comment about uploading files to the public directory. I'm not sure if it is worth a deeper technical explanation as well.

docs/basic-features/static-file-serving.md

@@ -23,3 +23,5 @@ This folder is also useful for `robots.txt`, Google Site Verification, and any o
 > **Note**: Be sure to not have a static file with the same name as a file in the `pages/` directory, as this will result in an error.
 >
 > Read more: <http://err.sh/next.js/conflicting-public-file-page>
+
+> **Note**: Only assets that are in the `public` directory at [build time](/docs/api-reference/cli.md#build) will be served by Next.js. Files added at runtime won't be available. We recommend using a third party service like [AWS S3](https://aws.amazon.com/s3/) for persistent file storage.