swiotlb: add documentation and rename swiotlb_do_find_slots()

Add some kernel-doc comments and move the existing documentation of struct
io_tlb_slot to its correct location. The latter was forgotten in commit
942a8186 ("swiotlb: move struct io_tlb_slot to swiotlb.c").

Use the opportunity to give swiotlb_do_find_slots() a more descriptive name
and make it clear how it differs from swiotlb_find_slots().

Signed-off-by: Petr Tesarik <petr.tesarik.ext@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>