dma-mapping: fix the kerneldoc for dma_map_sg_attrs

Add the missing description for the nents parameter, and fix a trivial
misalignment.

Fixes: fffe3cc8 ("dma-mapping: allow map_sg() ops to return negative error codes")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Christoph Hellwig <hch@lst.de>