dma-buf: minor doc touch-ups

Just some tiny edits:
- fix link to struct dma_fence
- give slightly more meaningful title - the polling here is about
  implicit fences, explicit fences (in sync_file or drm_syncobj) also
  have their own polling

v2: I misplaced the .rst include change corresponding to this patch.

Reviewed-by: Thomas Hellstrom <thomas.hellstrom@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200612070535.1778368-1-daniel.vetter@ffwll.ch

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