Document the new upstream OAuth 2.0 options

[sandhose]

We've introduced a few new options that needs to be documented in the following PRs:

• Allow setting an explicit upstream account name #3600
• Make the id_token optional on upstream OAuth 2.0 providers #3591
• Support Sign in with Apple #3521
• Allow fetching user claims through the userinfo_endpoint for upstream OAuth 2.0 providers #3363

[sandhose]

Fixed by #3707