Add explaination for q_ascend and q_descend

Add supllementary explainations for q_ascend and q_descend,
indicating the required memory operations that must be
implemented in these functions.

queue.h

@@ -207,6 +207,8 @@ void q_sort(struct list_head *head, bool descend);
  *
  * No effect if queue is NULL or empty. If there has only one element, do
  * nothing.
+ * The memory allocated to nodes that are removed must be freed after their
+ * removal.
  *
  * Reference:
  * https://leetcode.com/problems/remove-nodes-from-linked-list/
@@ -222,6 +224,8 @@ int q_ascend(struct list_head *head);
  *
  * No effect if queue is NULL or empty. If there has only one element, do
  * nothing.
+ * The memory allocated to nodes that are removed must be freed after their
+ * removal.
  *
  * Reference:
  * https://leetcode.com/problems/remove-nodes-from-linked-list/