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>

include/uapi/drm/v3d_drm.h

@@ -58,6 +58,11 @@ struct drm_v3d_submit_cl {
 	 * coordinate shader to determine where primitives land on the screen,
 	 * then writes out the state updates and draw calls necessary per tile
 	 * to the tile allocation BO.
+	 *
+	 * This BCL will block on any previous BCL submitted on the
+	 * same FD, but not on any RCL or BCLs submitted by other
+	 * clients -- that is left up to the submitter to control
+	 * using in_sync_bcl if necessary.
 	 */
 	__u32 bcl_start;
 
@@ -69,6 +74,11 @@ struct drm_v3d_submit_cl {
 	 * This is the second set of commands executed, which will either
 	 * execute the tiles that have been set up by the BCL, or a fixed set
 	 * of tiles (in the case of RCL-only blits).
+	 *
+	 * This RCL will block on this submit's BCL, and any previous
+	 * RCL submitted on the same FD, but not on any RCL or BCLs
+	 * submitted by other clients -- that is left up to the
+	 * submitter to control using in_sync_rcl if necessary.
 	 */
 	__u32 rcl_start;