Add better documentation of how S3 file upload/download works

[vincanger]

There are many different implementations for file upload, but we don't want to assume any on part of the user.

We should document better what process currently exists in the File Upload template code, with specific mention of what's happening in the getDownloadFileSignedURL and that it is accessible by any user in the app (if they know the file key). Here, it's important to mention that some apps may want files uploaded by specific users to be accesible to all users of the app, while other apps will want to keep those private and only accessible by the user who uploaded them.