DMA-API.txt: fix description of pci_map_sg/dma_map_sg scatterlists handling

- pci_map_sg/dma_map_sg are used with a scatter gather list that doesn't
  come from the block layer (e.g.  some network drivers do).

- how IOMMUs merge adjacent elements of the scatter/gather list is
  independent of how the block layer determines sees elements.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Tejun Heo <htejun@gmail.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/DMA-API.txt

@@ -316,12 +316,10 @@ reduce current DMA mapping usage or delay and try again later).
 	pci_map_sg(struct pci_dev *hwdev, struct scatterlist *sg,
 		int nents, int direction)
 
-Maps a scatter gather list from the block layer.
-
 Returns: the number of physical segments mapped (this may be shorter
-than <nents> passed in if the block layer determines that some
-elements of the scatter/gather list are physically adjacent and thus
-may be mapped with a single entry).
+than <nents> passed in if some elements of the scatter/gather list are
+physically or virtually adjacent and an IOMMU maps them with a single
+entry).
 
 Please note that the sg cannot be mapped again if it has been mapped once.
 The mapping process is allowed to destroy information in the sg.