dma-buf/sync_file: Surface sync-file uABI

We had all of the internal driver APIs, but not the all important
userspace uABI, in the dma-buf doc.  Fix that.  And re-arrange the
comments slightly as otherwise the comments for the ioctl nr defines
would not show up.

v2: Fix docs build warning coming from newly including the uabi header
    in the docs build

Signed-off-by: Rob Clark <robdclark@chromium.org>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>