drm/uapi: Document CTM matrix better

Document in which order the CTM matrix elements are stored.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230411222931.15127-2-ville.syrjala@linux.intel.comReviewed-by: Xaver Hugl <xaver.hugl@gmail.com>
Acked-by: Simon Ser <contact@emersion.fr>

include/uapi/drm/drm_mode.h

@@ -834,6 +834,11 @@ struct drm_color_ctm {
 	/*
 	 * Conversion matrix in S31.32 sign-magnitude
 	 * (not two's complement!) format.
+	 *
+	 * out   matrix    in
+	 * |R|   |0 1 2|   |R|
+	 * |G| = |3 4 5| x |G|
+	 * |B|   |6 7 8|   |B|
 	 */
 	__u64 matrix[9];
 };