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 <[e-mail]>
Cc: James Bottomley <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/DMA-API.txt

···
 	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.