dma-buf/sync_file: improve Kconfig description for Sync Files

We've got a complaint saying that the description was quite obtuse and
indeed it was. This patch tries to improve it.

Cc: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>