ptr_ring: clean up documentation

The only function safe to call without locks
is __ptr_ring_empty. Move documentation about
lockless use there to make sure people do not
try to use __ptr_ring_peek outside locks.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>