Cluster: DomainAdministratorCredential requires UPN format

[ThomasNieto]

Please document that DomainAdministratorCredential requires UPN format since you're using this method to impersonate.

[ThomasNieto]

Ideally the method would flex for <user> or @ syntax and if the former is used it converts it into the latter for use in the method so user does not have to change.