Document that passkeys can't work on macOS without OAuth2 somewhere :-) (see bug 2047836: OAuth2 external browser flow privacy considerations (should ask before launching))

[rtanglao]

• bug 2047836: OAuth2 external browser flow privacy considerations (should ask before launching)) #211
• from Bug 1864920: Enable passkey restricted entitlement via macOS provisioning profile for Thunderbird : Thunderbird doesn't meet the requirements of a web browser which is a requirement to obtain the com.apple.developer.web-browser.public-key-credential entitlement, Apple referred us to ASWebAuthenticationSession. <-- now that we are not using a tab in Thunderbird and instead using the system web browser this bug is no longer an issue as of Thunderbird 153