drm/v3d: Add some better documentation of the in_sync arguments.

Since this is UAPI, it's good to document what exactly the guarantees
we're providing are.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180928232126.4332-3-eric@anholt.net


Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>