[css-round-display] Add auto value to polar-origin

Author: Jihye Hong

css-round-display/Overview.bs

@@ -477,7 +477,12 @@ By 'polar-distance' property, a position of an element is specified based on the
 <p>
 The 'polar-origin' property sets a point of origin for polar coordinate system.
 <br><br>
-Not only elements are positioned within the containing block according to the origin but also properties such as display, margin, and padding are affected by the origin. In conventional coordinate system, the origin of coordinate system implicitly set to upper left corner of the containing block. But in polar coordinates, it is general to specify the point of origin as the center point of the containing block. Also, there would be a situation that the other point in the containing block area need to be the origin.
+Not only elements are positioned within the containing block according to the origin but also properties such as display, margin, and padding are affected by the origin. In conventional coordinate system, the origin of coordinate system set to the upper left corner of the containing block by default.
+But in polar coordinates, it is general to specify the point of origin as the center point of the containing block. Also, there would be a situation that the other point in the containing block area need to be the origin.
+
+Therefore, an element is positioned to the horizontally and vertically center in the area of the containing block when auto value is given to the polar-origin.
+If top, right, bottom, or left is used for positioning the element, auto value makes the same result of ‘top left’. In that case, the element can be positioned in the normal layout.
+
 </p>
 <pre class='propdef'>
     Name: polar-origin