DMA-API-HOWTO: <asm/generic/scatterlist.h> is no more

So don't mention it.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/DMA-API-HOWTO.txt

@@ -931,10 +931,8 @@ to "Closing".
 
 1) Struct scatterlist requirements.
 
-   Don't invent the architecture specific struct scatterlist; just use
-   <asm-generic/scatterlist.h>. You need to enable
-   CONFIG_NEED_SG_DMA_LENGTH if the architecture supports IOMMUs
-   (including software IOMMU).
+   You need to enable CONFIG_NEED_SG_DMA_LENGTH if the architecture
+   supports IOMMUs (including software IOMMU).
 
 2) ARCH_DMA_MINALIGN