Documentation: add doc for sync_file_get_fence()

Document the new function added to sync_file.c

v2: Adapt to fence_array

v3: Take in Chris Wilson suggestions

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>