Consider adding docs for Github Oauth2 multi-server usecase

[onobc]

Motivation

          Thank you both for your detailed explanations and sharing w/ one another (and the whole community by posting these details in the issue). 

We will see about adding some info about this use case to the docs to help future users not go through the exercise you did. Again, thank you.

Originally posted by @onobc in #5314 (comment)