doc/dev/cephx_protocol: fix couple errors

doc/dev/cephx_protocol: fix couple errors

there are 3 phases of authentication and authorization in cephx:

0. proto = CEPH_AUTH_UNKNOWN, for start the session,
   create auth handlers on both sides.
1. GET_AUTH_SESSION_KEY, to get authenticated, the client will
   get the session ticket at the end of this phase. the session ticket,
   is actually also a service ticket for CEPH_ENTITY_TYPE_AUTH.
2. CEPHX_GET_PRINCIPAL_SESSION_KEY, to get authorized, the client
   will get the requested, service tickets at the end of this phase.

Signed-off-by: Kefu Chai <kchai@redhat.com>