Remote Overlay requires upload-redirect=true

[qqmyers]

What steps does it take to reproduce the issue?
As of ~#8923, the RemoteOverlay store requires the upload-redirect=true setting and this is not noted in the guides. Nominally RemoteOverlay stores always require this as they only work by using the direct upload form of the /add call to refer to the remote file, so there are two options:

1. document the requirement
2. change the code to always assume RemoteOverlay stores are direct upload enabled and ignore any upload-redirect configuration for such stores (and/or flag its use with a warning in the log).