dma-buf: Documentation for buffer sharing framework

Add documentation for dma buffer sharing framework, explaining the
various operations, members and API of the dma buffer sharing
framework.

Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Sumit Semwal <sumit.semwal@ti.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>