[css-shapes-1] Restore the default value of <radial-size> when omitted

Author: cdoublev

css-shapes-1/Overview.bs

@@ -512,10 +512,10 @@ Supported Shapes</h3>
 				the circle's radius.
 				Rather than referring to the [=gradient box=],
 				values are resolved against the [=reference box=].
+				Two <<length-percentage>> values are invalid.
+				If omitted it defaults to ''<radial-size>/closest-side''.
 
-			* Two <<length-percentage>> values are invalid.
-
-				The <<position>> argument defines
+			* The <<position>> argument defines
 				the center of the circle.
 				Unless otherwise specified,
 				this defaults to <css>center</css> if omitted.
@@ -532,6 +532,7 @@ Supported Shapes</h3>
 				the horizontal and vertical radiuses of the ellipse.
 				Rather than referring to the [=gradient box=],
 				values are resolved against the [=reference box=].
+				If omitted it defaults to ''<radial-size>/closest-side''.
 
 			* The <<position>> argument defines
 				the center of the ellipse.