net/gnrc/pktbuf.h: add note in gnrc_pktbuf_mark

sys/include/net/gnrc/pktbuf.h

@@ -113,6 +113,9 @@ gnrc_pktsnip_t *gnrc_pktbuf_add(gnrc_pktsnip_t *next, const void *data, size_t s
  * @param[in] size  The size of the new packet snip.
  * @param[in] type  The type of the new packet snip.
  *
+ * @note    It's not guaranteeed that `result->data` points to the same address
+ *          as the original `pkt->data`.
+ *
  * @return  The new packet snip in @p pkt on success.
  * @return  NULL, if pkt == NULL or size == 0 or size > pkt->size or pkt->data == NULL.
  * @return  NULL, if no space is left in the packet buffer.