DMA-API.txt: add dma_sync_single/sg API description

This adds the description of the following eight function:

dma_sync_single_for_cpu
pci_dma_sync_single_for_cpu
dma_sync_single_for_device
pci_dma_sync_single_for_device
dma_sync_sg_for_cpu
pci_dma_sync_sg_for_cpu
dma_sync_sg_for_device
pci_dma_sync_sg_for_device

It was unclear that the API permits a partial sync (some network drivers
already do though).  I made it clear that the sync_single API can do a
partial sync but the sync_sg API can't.

We could do a partial sync with the sync_sg API too, however, it's
difficult for driver writers to correctly use the sync_sg API for a
partial sync since the scatterlists passed in to the mapping API can't be
modified.  It's unlikely that driver writers want to do a partial sync
with the sync_sg API (because the sync_sg API are usually used for block
drivers).  So I think that it's better to forbid a partial sync with the
sync_sg API.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: David Miller <davem@davemloft.net>
Acked-by: James Bottomley <James.Bottomley@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>