doc: fix broken references

There are numerous broken references to Documentation files (in other
Documentation files, in comments, etc.). These broken references are
caused by typo's in the references, and by renames or removals of the
Documentation files. Some broken references are simply odd.

Fix these broken references, sometimes by dropping the irrelevant text
they were part of.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/arch/x86/include/asm/dma-mapping.h b/arch/x86/include/asm/dma-mapping.h
index d4c419f..ed3065f 100644
--- a/arch/x86/include/asm/dma-mapping.h
+++ b/arch/x86/include/asm/dma-mapping.h
@@ -2,7 +2,7 @@
 #define _ASM_X86_DMA_MAPPING_H
 
 /*
- * IOMMU interface. See Documentation/PCI/PCI-DMA-mapping.txt and
+ * IOMMU interface. See Documentation/DMA-API-HOWTO.txt and
  * Documentation/DMA-API.txt for documentation.
  */