drm/blend: Define the direction of Z position values

We forgot that.

Proof is the one igt testcase we have:

https://gitlab.freedesktop.org/drm/igt-gpu-tools/blob/master/tests/kms_atomic.c#L280

While at it also document that we have immutable zpos properties in
some cases.
Reported-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190906144459.16025-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_blend.c

@@ -130,7 +130,12 @@
  *	Z position is set up with drm_plane_create_zpos_immutable_property() and
  *	drm_plane_create_zpos_property(). It controls the visibility of overlapping
  *	planes. Without this property the primary plane is always below the cursor
- *	plane, and ordering between all other planes is undefined.
+ *	plane, and ordering between all other planes is undefined. The positive
+ *	Z axis points towards the user, i.e. planes with lower Z position values
+ *	are underneath planes with higher Z position values. Note that the Z
+ *	position value can also be immutable, to inform userspace about the
+ *	hard-coded stacking of overlay planes, see
+ *	drm_plane_create_zpos_immutable_property().
  *
  * pixel blend mode:
  *	Pixel blend mode is set up with drm_plane_create_blend_mode_property().