Update TODO, cosmetic changes, spelling -- US English spells it canceled and canceling vs cancelled and cancelling. No idea why.

sched/pthread/pthread_condwait.c

@@ -107,7 +107,11 @@ int pthread_cond_wait(FAR pthread_cond_t *cond, FAR pthread_mutex_t *mutex)
       ret |= pthread_takesemaphore((FAR sem_t *)&cond->sem);
       sched_unlock();
 
-      /* Reacquire the mutex */
+      /* Reacquire the mutex.
+       *
+       * REVISIT: When cancellation points are enabled, we will almost
+       * certainly hold the mutex when the pthread is canceled.
+       */
 
       sinfo("Reacquire mutex...\n");
       ret |= pthread_takesemaphore((FAR sem_t *)&mutex->sem);

sched/pthread/pthread_mutexdestroy.c

@@ -75,7 +75,7 @@ int pthread_mutex_destroy(FAR pthread_mutex_t *mutex)
 
   sinfo("mutex=0x%p\n", mutex);
 
-  if (!mutex)
+  if (mutex == NULL)
     {
       ret = EINVAL;
     }
@@ -100,7 +100,7 @@ int pthread_mutex_destroy(FAR pthread_mutex_t *mutex)
          ret = kill(mutex->pid, 0);
          if (ret < 0)
            {
-             /* That thread associated with the PID no longer exists */
+             /* The thread associated with the PID no longer exists */
 
              mutex->pid = -1;