user.name and user.displayName usage

[timcappalli]

Summary

• Document best practices for user.name and user.displayName
• Document differences in client behavior

Originally discussed in: w3c/webauthn#1852

[timcappalli]

https://github.com/passkeydeveloper/passkeys.dev/wiki/Implement:-Back-End-Requirements

[MasterKale]

I added some clarifications on user.name vs user.displayName in w3c/webauthn#1932 which we can use over here:

• https://w3c.github.io/webauthn/#dom-publickeycredentialentity-name
• https://w3c.github.io/webauthn/#dom-publickeycredentialuserentity-displayname