drm: Clarify DRM_MODE_REFLECT_X/Y documentation

DRM_MODE_REFLECT_X and DRM_MODE_REFLECT_Y meaning seems a bit unclear
to me, so try to clarify that with a bit of ascii graphics.

Changes since v1:
  - Move the ascii graphics in the kerneldoc where all plane
    properties are already documented and make sure it's properly
    rendered, suggestested by Daniel Vetter.
Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180910172946.18539-1-alexandru-cosmin.gheorghe@arm.com

drivers/gpu/drm/drm_blend.c

@@ -101,6 +101,28 @@
  *	Without this property the rectangle is only scaled, but not rotated or
  *	reflected.
  *
+ *	Possbile values:
+ *
+ *	"rotate-<degrees>":
+ *		Signals that a drm plane is rotated <degrees> degrees in counter
+ *		clockwise direction.
+ *
+ *	"reflect-<axis>":
+ *		Signals that the contents of a drm plane is reflected along the
+ *		<axis> axis, in the same way as mirroring.
+ *
+ *	reflect-x::
+ *
+ *			|o |    | o|
+ *			|  | -> |  |
+ *			| v|    |v |
+ *
+ *	reflect-y::
+ *
+ *			|o |    | ^|
+ *			|  | -> |  |
+ *			| v|    |o |
+ *
  * zpos:
  *	Z position is set up with drm_plane_create_zpos_immutable_property() and
  *	drm_plane_create_zpos_property(). It controls the visibility of overlapping

include/uapi/drm/drm_mode.h

@@ -186,8 +186,9 @@ extern "C" {
 /*
  * DRM_MODE_REFLECT_<axis>
  *
- * Signals that the contents of a drm plane is reflected in the <axis> axis,
+ * Signals that the contents of a drm plane is reflected along the <axis> axis,
  * in the same way as mirroring.
+ * See kerneldoc chapter "Plane Composition Properties" for more details.
  *
  * This define is provided as a convenience, looking up the property id
  * using the name->prop id lookup is the preferred method.