auth/CephxClientHandler: explain why skipping extra_tickets is needed

auth/CephxClientHandler: explain why skipping extra_tickets is needed

Since nautilus, CEPHX_GET_AUTH_SESSION_KEY shares both the auth ticket
and the service tickets, sparing an extra round-trip to get the service
tickets via CEPHX_GET_PRINCIPAL_SESSION_KEY.  This applies to msgr1 as
well, but we don't take advantage of it on the client side.

However, fixing CephxClientHandler to do the right thing breaks msgr1.
Since msgr1 is on its way out, rather than also fixing MonClient just
document the bug and the current behaviour.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>