dma-mapping: document dma_{alloc,free}_pages

Document the new dma_alloc_pages and dma_free_pages APIs, and fix
up the documentation for dma_alloc_noncoherent and dma_free_noncoherent.

Reported-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>