proxy config options for CORS lacking documentation

The docs CORS page [1] only talks about users that want to configure CORS options per container

There's some options in the proxy app for the operator to configure global CORS support (like a single page app hosted somewhere else that makes requests directly to a swift)

cors_allow_origin, strict_cors_mode and cors_expose_headers are mentioned in the deployment guide [2] - but they should be on the CORS page as well I think, with some prose describing the use-case and how an operator can configure/use them.

1. https://docs.openstack.org/developer/swift/deployment_guide.html
2. https://docs.openstack.org/developer/swift/deployment_guide.html#proxy-server-configuration