Using "username" for user's name with SAML authentication

Dear maintainers,

When using LDAP as an authentication backend a `displayName` option is available and is displayed in the top right corner next to the user's avatar.
When using SAML such alternative doesn't exist and the code defaults to displaying the `username` attribute.
The documentation doesn't currently state what properties `username` needs to meet nor if this field needs to be unique between users.

Could someone clarify if:
1. `username` can contain non unique identifiers.
2. if `username` can containing non-ASCII characters
3. using a name such as "John Doe" is acceptable

Thanks