define blur radius's semantics, disallow negative values

Author: fantasai

css3-text/Overview.src.html

@@ -2653,9 +2653,12 @@ <h3 id="text-shadow"><span class="secno">8.3</span>
       shadow above the text.</p>
 
     <p>A blur radius may optionally be specified after the shadow offset.
-      The blur radius is a length value that indicates the boundaries of the
-      blur effect. The exact algorithm for computing the blur effect is not
-      specified. If the blur radius is not specified, it is equal to zero.</p>
+      If the blur radius is not specified, it is equal to zero. The blur
+      radius is a length value that indicates the boundaries of the blur
+      effect. If it is zero, the shadow is sharp; otherwise the larger
+      the value, the more the shadow is blurred.  The exact algorithm for
+      computing the blur effect is not specified. Negative values are not
+      allowed.</p>
 
     <p>A color value may optionally be specified before or after the length
       values of the shadow effect. This color value must be used as the color