Documentation/core-api: add swiotlb documentation

There's currently no documentation for the swiotlb. Add documentation
describing usage scenarios, the key APIs, and implementation details.
Group the new documentation with other DMA-related documentation.
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Petr Tesarik <petr@tesarici.cz>
Signed-off-by: Christoph Hellwig <hch@lst.de>

Documentation/core-api/index.rst

@@ -102,6 +102,7 @@ more memory-management documentation in Documentation/mm/index.rst.
    dma-api-howto
    dma-attributes
    dma-isa-lpc
+   swiotlb
    mm-api
    genalloc
    pin_user_pages