drm/prime: Clarify DMA-BUF/GEM Object lifetime

From the description of the "DMA-BUF/GEM Object references
and lifetime overview" it is not clear when exactly
dma_buf gets destroyed and memory freed: only driver
.release function mentioned which makes confusion on the
real buffer's lifetime.

Add more description so all the paths are covered.

Cc: Rob Clark <robdclark@gmail.com>
Cc: Dave Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
[danvet: Minor spelling fixes, and some clarification of the 2nd
paragraph.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1485500665-27690-1-git-send-email-andr2000@gmail.com