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: [email protected]
Cc: Jonathan Corbet <[email protected]>
Cc: Sumit Semwal <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Signed-off-by: Sumit Semwal <[email protected]>
  [sumits: fix cosmetic issues]
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

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