Remove "default domain" & restore unscoped token

- Removes mentions of a default domain (which is intended to be an
  implementation detail). Everything the document describes as a
  behavior related default-domain can be more simply achieved by having
  the client assume a default domain ID value of `default` to correspond
  with the current implementation (e.g. --os-domain-id=default), rather
  than specifying a one-off behavior at the API-level.
- The previous wording left no room for a token without a defined
  authorization scope

Change-Id: Idde45a884dd56748bb3f1409116e7783bdd1a2ed