Doc: DMA-API update

Fix typos and update function parameters.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Muli Ben-Yehuda <muli@il.ibm.com>
Cc: James Bottomley <James.Bottomley@steeleye.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>