swiotlb: fix kernel-doc description for swiotlb_del_transient

Describe the pool argument in the kernel-doc comment for
swiotlb_del_transient.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>