More comments on the ticket lock code

include/ucore/ticket_lock.h

@@ -3,7 +3,9 @@
 
 #include <pthread.h>
 
-/** An fair alternative to a bare pthread_mutex_t to prevent starvation*/
+/** An fair alternative to a bare pthread_mutex_t to prevent starvation.
+ * The members of UCTicketLock must never be accessed directly.
+ */
 struct UCTicketLock {
     pthread_cond_t cond;
     pthread_mutex_t mutex;