net: hns3: add comments for hclge_dbg_fill_content()

When we use hclge_dbg_fill_content() to fill contents with
specific format according to struct hclge_dbg_item *items,
it may cause content cover due to unreasonable items.

So add comments to explain how to avoid it.

Signed-off-by: Hao Chen <chenhao288@hisilicon.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>