add docs for server.middleware =, update transition copy

[osi-jehrlich]

Documentation to match PR miragejs/miragejs#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.