webui: document "remote_user=allowed" policy for any realm
webui: document "remote_user=allowed" policy for any realm

See https://community.privacyidea.org/t/remote-user-sso-with-kerberos/469/3 for details
privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea