Problem

Some documentation for the upstream configuration option provider_slug, and the global sso_proxy
DefaultProviderSlug option is missing

Solution

Add in the missing documentation.

This is annoying. Do we want to force merge?

@jphines Actually, I think that might be a good idea. I've tried to dig into why it's suggesting this change in code coverage, but unclear at the moment.

If you're happy with the change, let's force merge 👍