Update singleton_sema_priority_greater function description for clarity

src/threads/synch.c

@@ -270,8 +270,11 @@ cond_init (struct condition *cond)
    semaphore is greater than the priority of the only thread in the second
    singleton semaphore.
    
-   If used for insertion, the third argument must be a pointer to struct
-   list_elem for the thread being inserted, otherwise must be NULL. */
+   Where this function is used for insertion in a singleton semaphore list, the
+   third argument may specify a list_elem * to assume corresponds to the thread
+   waiting for the inserting semaphore. For correctness, ensure this thread
+   calls sema_down () for this semaphore before future list accesses. */
+
 static bool
 singleton_sema_priority_greater (const struct list_elem *a, 
                                  const struct list_elem *b,