Adjust diagram to include variant where End-user enters OP (instead of WebID) on the client.

[elf-pavlik]

requested by @namedgraph in https://gitter.im/solid/authentication-panel?at=632c12a7f4d7a323dee54c94

the conversation includes details of two small sequences which need to be made alt/opt based on what the end-user entered on the client

mermaid supports it with https://mermaid-js.github.io/mermaid/#/sequenceDiagram?id=alt