Split the discussion of v2 authentication into two parts:
1) A specification of the handshake between the client, registry and
authentication service.
2) A description of how `docker/distribution` implements this using JWT.
This should make it clearer that `#2` is an implementation detail, and
that clients should regard tokens as opaque entities that only the
registry and authentication service should understand.
Signed-off-by: Matt Moore <mattmoor@google.com>
