add docs to queue_read and stack_pop

lab01/zad1/include/queue.h

@@ -10,6 +10,7 @@ Queue *queue_new(size_t length, size_t element_size);
 
 void queue_free(Queue *queue);
 
+// The caller must free the return value after use.
 void *queue_read(Queue *queue);
 
 bool queue_write(Queue *queue, void *value);

lab01/zad1/include/stack.h

@@ -10,6 +10,7 @@ Stack *stack_new(size_t size, size_t element_size);
 
 void stack_free(Stack *stack);
 
+// The caller must free the return value after use.
 void *stack_pop(Stack *stack);
 
 bool stack_push(Stack *stack, void *value);