dma-buf: Update cpu access documentation

- Again move the information relevant for driver writers next to the
  callbacks.
- Put the overview and userspace interface documentation into a DOC:
  section within the code.
- Remove the text that mmap needs to be coherent - since the
  DMA_BUF_IOCTL_SYNC landed that's no longer the case. But keep the text
  that for pte zapping exporters need to adjust the address space.
- Add a FIXME that kmap and the new begin/end stuff used by the SYNC
  ioctl don't really mix correctly. That's something I just realized
  while doing this doc rework.
- Augment function and structure docs like usual.

Cc: linux-doc@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
  [sumits: fix cosmetic issues]
Link: http://patchwork.freedesktop.org/patch/msgid/20161209185309.1682-5-daniel.vetter@ffwll.ch