add docs for `server.middleware =`, update transition copy

[osi-jehrlich]

Documentation to match PR https://github.com/miragejs/miragejs/pull/1089 which adds the server.middleware = API.

[osi-jehrlich]

Of note: I ran into quite a few yarn install issues with older packages not being installable on darwin-arm64 so I was unable to run tests locally. I'm hoping that these changes are all okay. I did get the local dev server running, though.